mailbox: pcc: Fix crash when request PCC channel 0
Hoan Tran authored
When PCCT is not available, kernel crashes as below when requests PCC
channel 0. This patch fixes this issue.

[    0.920454] PCCT header not found.
...
[    8.031309] Unable to handle kernel NULL pointer dereference at virtual address 00000010
[    8.031310] [0000000000000010] user address but active_mm is swapper
[    8.031312] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[    8.031313] Modules linked in:
[    8.031316] CPU: 31 PID: 1 Comm: swapper/0 Tainted: G        W       4.13.0-rc1 #18
[    8.031317] Hardware name: AppliedMicro(R) 07/20/2017
[    8.031318] task: ffff809ef3b08000 task.stack: ffff809ef3b10000
[    8.031322] PC is at pcc_mbox_request_channel+0x8c/0x160
[    8.031325] LR is at xgene_slimpro_i2c_probe+0x1c0/0x378
[    8.031326] pc : [<ffff000008899450>] lr : [<ffff000008819dac>] pstate: 00000045
[    8.031327] sp : ffff809ef3b13bd0
[    8.031327] x29: ffff809ef3b13bd0 x28: ffff000008ed90a0
[    8.031329] x27: ffff000009091000 x26: ff...
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