rtl8187: Fix NULL pointer dereference in priv->conf_mutex

rtl8187: Fix NULL pointer dereference in priv->conf_mutex

This can be reproduced by bind/unbind the driver multiple times
in AM3517 board.

Analysis revealed that rtl8187_start() was invoked before probe
finishes(ie. before the mutex is initialized).

 INFO: trying to register non-static key.
 the code is fine but needs lockdep annotation.
 turning off the locking correctness validator.
 CPU: 0 PID: 821 Comm: wpa_supplicant Not tainted 4.9.80-dirty #250
 Hardware name: Generic AM3517 (Flattened Device Tree)
 [<c010e0d8>] (unwind_backtrace) from [<c010beac>] (show_stack+0x10/0x14)
 [<c010beac>] (show_stack) from [<c017401c>] (register_lock_class+0x4f4/0x55c)
 [<c017401c>] (register_lock_class) from [<c0176fe0>] (__lock_acquire+0x74/0x1938)
 [<c0176fe0>] (__lock_acquire) from [<c0178cfc>] (lock_acquire+0xfc/0x23c)
 [<c0178cfc>] (lock_acquire) from [<c08aa2f8>] (mutex_lock_nested+0x50/0x3b0)
 [<c08aa2f8>] (mutex_lock_nested) from [<c05f5bf8>] (rtl8187_start+0x2c/0xd54)
 [<c05f5bf8>] (rtl8187_start) from [<c082dea0>] (drv_start+0xa8/0x320)
 [<c082dea0>] (drv_start) from [<c084d1d4>] (ieee80211_do_open+0x2bc/0x8e4)
 [<c084d1d4>] (ieee80211_do_open) from [<c069be94>] (__dev_open+0xb8/0x120)
 [<c069be94>] (__dev_open) from [<c069c11c>] (__dev_change_flags+0x88/0x14c)
 [<c069c11c>] (__dev_change_flags) from [<c069c1f8>] (dev_change_flags+0x18/0x48)
 [<c069c1f8>] (dev_change_flags) from [<c0710b08>] (devinet_ioctl+0x738/0x840)
 [<c0710b08>] (devinet_ioctl) from [<c067925c>] (sock_ioctl+0x164/0x2f4)
 [<c067925c>] (sock_ioctl) from [<c02883f8>] (do_vfs_ioctl+0x8c/0x9d0)
 [<c02883f8>] (do_vfs_ioctl) from [<c0288da8>] (SyS_ioctl+0x6c/0x7c)
 [<c0288da8>] (SyS_ioctl) from [<c0107760>] (ret_fast_syscall+0x0/0x1c)
 Unable to handle kernel NULL pointer dereference at virtual address 00000000
 pgd = cd1ec000
 [00000000] *pgd=8d1de831, *pte=00000000, *ppte=00000000
 Internal error: Oops: 817 [#1] PREEMPT ARM
 Modules linked in:
 CPU: 0 PID: 821 Comm: wpa_supplicant Not tainted 4.9.80-dirty #250
 Hardware name: Generic AM3517 (Flattened Device Tree)
 task: ce73eec0 task.stack: cd1ea000
 PC is at mutex_lock_nested+0xe8/0x3b0
 LR is at mutex_lock_nested+0xd0/0x3b0

Cc: stable@vger.kernel.org
Signed-off-by: Sudhir Sreedharan <ssreedharan@mvista.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>