brcmfmac: Fix race condition between USB probe/load and disconnect.

When a USB device gets disconnected due to for example removal
then it is possible that it is still in the loading phase due to
the asynchronous load routines. These routines can then possible
access memory which has been freed. Fix this by mutex locking the
device init phase.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>