mac80211: brcmfmac: fix a possible NULL pointer dereference

mac80211: brcmfmac: fix a possible NULL pointer dereference

This fixes a possible crash in the brcmf_fw_request_nvram_done():
[   31.687293] Backtrace:
[   31.689760] [<c004fb4c>] (__wake_up_common) from [<c004fc38>] (__wake_up_locked+0x1c/0x24)
[   31.698043]  r10:c6794000 r9:00000009 r8:00000001 r7:bf54dda0 r6:a0000013 r5:c78e7d38
[   31.705928]  r4:c78e7d3c r3:00000000
[   31.709528] [<c004fc1c>] (__wake_up_locked) from [<c00502a8>] (complete+0x3c/0x4c)
[   31.717148] [<c005026c>] (complete) from [<bf54590c>] (brcmf_fw_request_nvram_done+0x5c8/0x6a4 [brcmfmac])
[   31.726818]  r7:bf54dda0 r6:c6794000 r5:00001990 r4:c6782380
[   31.732544] [<bf545344>] (brcmf_fw_request_nvram_done [brcmfmac]) from [<c0204e40>] (request_firmware_work_func+0x38/0x60)
[   31.743607]  r10:00000008 r9:c6bdd700 r8:00000000 r7:c72c3cd8 r6:c67f4300 r5:c6bda300
[   31.751493]  r4:c67f4300
[   31.754046] [<c0204e08>] (request_firmware_work_func) from [<c0034458>] (process_one_work+0x1e0/0x318)
[   31.763365]  r4:c72c3cc0
[   31.765913] [<c0034278>] (process_one_work) from [<c0035234>] (worker_thread+0x2f4/0x448)
[   31.774107]  r10:00000008 r9:00000000 r8:c6bda314 r7:c72c3cd8 r6:c6bda300 r5:c6bda300
[   31.781993]  r4:c72c3cc0
[   31.784545] [<c0034f40>] (worker_thread) from [<c003984c>] (kthread+0x100/0x114)
[   31.791949]  r10:00000000 r9:00000000 r8:00000000 r7:c0034f40 r6:c72c3cc0 r5:00000000
[   31.799836]  r4:c735dc00 r3:c79ed540
[   31.803438] [<c003974c>] (kthread) from [<c00097d0>] (ret_from_fork+0x14/0x24)
[   31.810672]  r7:00000000 r6:00000000 r5:c003974c r4:c735dc00
[   31.816378] Code: e5b53004 e1a07001 e1a06002 e243000c (e5934000)
[   31.822487] ---[ end trace a0ffbb07a810d503 ]---

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 83bcacb5215c21e1894fbe3d651d83948479ce91)