Comment 0 for bug 1836215

SRU Justification:
------------------

Impact:
-------
      Kernel crash upon inserting shutdown or rebooting the device.

Test case:
----------
      1) Power up the device, Redpine modules are loaded by default.
      2) Reboot or shutdown the device.
      3) Observe the behavior.

Result:
-------
       Redpine driver crashes the entire kernel and below is the crash log.
       ... skipping ...
       [ 47.002030] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000038
       [ 47.010828] IP: rsi_shutdown+0x2c/0x100 [rsi_sdio]
       [ 47.016180] PGD 0 P4D 0
       [ 47.019025] Oops: 0000 1 SMP PTI
       [ 47.136275] CPU: 1 PID: 1 Comm: systemd-shutdow Not tainted
4.15.0-1035-oem #40-Ubuntu
       [ 47.145131] Hardware name: Dell Inc. Edge Gateway 3001/0YMHCG,
BIOS 01.00.08 10/23/2018
       [ 47.154096] RIP: 0010:rsi_shutdown+0x2c/0x100 [rsi_sdio]
       [ 47.160039] RSP: 0018:ffffb08d0000bd78 EFLAGS: 00010246
       [ 47.165887] RAX: 0000000000000000 RBX: ffffa08b77f1d000 RCX: 0000000000000000
       [ 47.173869] RDX: 0000000000000002 RSI: ffffffffc07aa77f RDI: 0000000000000001
       [ 47.181851] RBP: ffffb08d0000bd90 R08: 0000000000000000 R09: ffffb08d0000bc50
       [ 47.189834] R10: 0000000000000000 R11: 000000000000023a R12: ffffa08b6d940000
       [ 47.197816] R13: ffffa08b3cbfb408 R14: ffffffff8f556137 R15: ffffa08b3cbfb468
       [ 47.205801] FS: 00007f85cd093940(0000) GS:ffffa08b70b00000(0000)
knlGS:0000000000000000
       [ 47.214854] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
       [ 47.221281] CR2: 0000000000000038 CR3: 000000006a930000 CR4: 00000000001006e0
       [ 47.229262] Call Trace:
       [ 47.232017] device_shutdown+0x14e/0x200
       [ 47.236416] kernel_power_off+0x35/0x70
       [ 47.240711] SYSC_reboot+0x132/0x210
       [ 47.244721] ? SYSC_rt_sigtimedwait+0x88/0xe0
       [ 47.249604] ? do_writev+0x61/0xf0
       [ 47.253411] ? do_writev+0x61/0xf0
       [ 47.257219] ? sigprocmask+0x6f/0xa0
       [ 47.261225] SyS_reboot+0xe/0x10
       [ 47.264841] do_syscall_64+0x73/0x130
       [ 47.268945] entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Root cause analysis:
--------------------
       Device operating mode 8(bluetooth alone), our driver supports multiple modes like 13(WiFi + Bluetooth), WiFi related structures are not been initialized in Bluetooth alone mode(8), dereferencing them in rsi_shutdown function causes crash. In Redpine
       driver, this NULL check is missed. Hence, Added the fix for the same.

Fix:
----
       Added proper NULL check in shutdown function.

Regression Petential:
---------------------
        This is a very direct issue Since the driver is crashing reboot. We rebooted multiples times, checked the driver stability. Every thing ran smooth.