rtlwifi: Fix use of mutex in interrupt code

rtlwifi: Fix use of mutex in interrupt code

A previous conversion from semaphoreto mutexes missed the fact that one
of the semaphores was used in interrupt code. Fixed by changing to
a spinlock.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>