This bug will cause NULL pointer after commit dfac17, and cause
wrong package in I2S DMA transfer before commit dfac17.

Tested on RK3288-pinky2 board.

Detail:
I2S DMA transfer(sound/core/pcm_dmaengine.c):
dmaengine_pcm_prepare_and_submit -->
dmaengine_prep_dma_cyclic -->
pl330_prep_dma_cyclic -->
the case:
1. pl330_submit_req(desc0): thrd->req[0].desc = desc0, thrd->lstenq = 0
2. pl330_submit_req(desc1): thrd->req[1].desc = desc1, thrd->lstenq = 1
3. _start(desc0) by submit_req: thrd->req_running = 0
   because: idx = 1 - thrd->lstenq = 0
4. pl330_update(desc0 OK): thrd->req[0].desc = NULL, desc0 to req_done list
   because: idx = active = thrd->req_running = 0
5. _start(desc1) by pl330_update: thrd->req_running = 1
   because:
   idx = 1 - thrd->lstenq = 0, but thrd->req[0].desc == NULL,
   so:
   idx = thrd->lstenq = 1
6. pl330_submit_req(desc2): thrd->req[0].desc = desc2, thrd->lstenq = 0
7. _start(desc1) by submit_req: thrd->req_running = 1
   because: idx = 1 - thrd->lstenq = 1
   Note: _start started the same descs
         _start should start desc2 here, NOT desc1

8. pl330_update(desc1 OK): thrd->req[1].desc = NULL, desc1 to req_done list
   because: idx = active = thrd->req_running = 1
9. _start(desc2) by pl330_update : thrd->req_running = 0
   because: idx = 1 - thrd->lstenq = 0
10.pl330_update(desc1 OK, NOT desc2): thrd->req[0].desc = NULL,
   desc2 to req_done list
   because: idx = active = thrd->req_running = 0

11.pl330_submit_req(desc3): thrd->req[0].desc = desc3, thrd->lstenq = 0
12.pl330_submit_req(desc4): thrd->req[1].desc = desc4, thrd->lstenq = 1
13._start(desc3) by submit_req: thrd->req_running = 0
   because: idx = 1 - thrd->lstenq = 0
14.pl330_update(desc2 OK NOT desc3): thrd->req[0].desc = NULL
   desc3 to req_done list
   because: idx = active = thrd->req_running = 0
15._start(desc4) by pl330_update: thrd->req_running = 1
   because:
   idx = 1 - thrd->lstenq = 0, but thrd->req[0].desc == NULL,
   so:
   idx = thrd->lstenq = 1
16.pl330_submit_req(desc5): thrd->req[0].desc = desc5, thrd->lstenq = 0
17._start(desc4) by submit_req: thrd->req_running = 1
   because: idx = 1 - thrd->lstenq = 1
18.pl330_update(desc3 OK NOT desc4): thrd->req[1].desc = NULL
   desc4 to req_done list
   because: idx = active = thrd->req_running = 1
19._start(desc4) by pl330_update: thrd->req_running = 0
   because:
   idx = 1 - thrd->lstenq = 1, but thrd->req[1].desc == NULL,
   so:
   idx = thrd->lstenq = 0
20.pl330_update(desc4 OK): thrd->req[0].desc = NULL, desc5 to req_done list
   because: idx = active = thrd->req_running = 0
21.pl330_update(desc4 OK):
   1) before commit dfac17(set req_running -1 in pl330_update/mark_free()):
      because: active = -1, abort
      result: desc0-desc5's callback are all called,
	      but step 10 and step 18 go wrong.
   2) before commit dfac17:
      idx = active = thrd->req_runnig = 0 -->
      descdone = thrd->req[0] = NULL -->
      list_add_tail(&descdone->rqd, &pl330->req_done); -->
      got NULL pointer!!!
Signed-off-by: Addy Ke <addy.ke@rock-chips.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>