ath9k: do not return early to fix rcu unlocking

Starting with commit d94a461d ("ath9k: use ieee80211_tx_status_noskb
where possible") the driver uses rcu_read_lock() && rcu_read_unlock(), yet on
returning early in ath_tx_edma_tasklet() the unlock is missing leading to stalls
and suspicious RCU usage:

 ===============================
 [ INFO: suspicious RCU usage. ]
 4.9.0-rc8 #11 Not tainted
 -------------------------------
 kernel/rcu/tree.c:705 Illegal idle entry in RCU read-side critical section.!

 other info that might help us debug this:

 RCU used illegally from idle CPU!
 rcu_scheduler_active = 1, debug_locks = 0
 RCU used illegally from extended quiescent state!
 1 lock held by swapper/7/0:
 #0:
  (
 rcu_read_lock
 ){......}
 , at:
 [<ffffffffa06ed110>] ath_tx_edma_tasklet+0x0/0x450 [ath9k]

 stack backtrace:
 CPU: 7 PID: 0 Comm: swapper/7 Not tainted 4.9.0-rc8 #11
 Hardware name: Acer Aspire V3-571G/VA50_HC_CR, BIOS V2.21 12/16/2013
  ffff88025efc3f38 ffffffff8132b1e5 ffff88017ede4540 0000000000000001
  ffff88025efc3f68 ffffffff810a25f7 ffff88025efcee60 ffff88017edebdd8
  ffff88025eeb5400 0000000000000091 ffff88025efc3f88 ffffffff810c3cd4
 Call Trace:
  <IRQ>
  [<ffffffff8132b1e5>] dump_stack+0x68/0x93
  [<ffffffff810a25f7>] lockdep_rcu_suspicious+0xd7/0x110
  [<ffffffff810c3cd4>] rcu_eqs_enter_common.constprop.85+0x154/0x200
  [<ffffffff810c5a54>] rcu_irq_exit+0x44/0xa0
  [<ffffffff81058631>] irq_exit+0x61/0xd0
  [<ffffffff81018d25>] do_IRQ+0x65/0x110
  [<ffffffff81672189>] common_interrupt+0x89/0x89
  <EOI>
  [<ffffffff814ffe11>] ? cpuidle_enter_state+0x151/0x200
  [<ffffffff814ffee2>] cpuidle_enter+0x12/0x20
  [<ffffffff8109a6ae>] call_cpuidle+0x1e/0x40
  [<ffffffff8109a8f6>] cpu_startup_entry+0x146/0x220
  [<ffffffff810336f8>] start_secondary+0x148/0x170
Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Fixes: d94a461d ("ath9k: use ieee80211_tx_status_noskb where possible")
Cc: <stable@vger.kernel.org> # v4.9
Acked-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Gabriel Craciunescu <nix.or.die@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -2713,7 +2713,7 @@ void ath_tx_edma_tasklet(struct ath_softc *sc)
 		fifo_list = &txq->txq_fifo[txq->txq_tailidx];
 		if (list_empty(fifo_list)) {
 			ath_txq_unlock(sc, txq);
-			return;
+			break;
 		}
 
 		bf = list_first_entry(fifo_list, struct ath_buf, list);