rt2x00: Fix unbalanced mutex locking

The usb_cache_mutex was not correctly released
under all circumstances. Both rt73usb as rt2500usb
didn't release the mutex under certain conditions
when the register access failed. Obviously such
failure would lead to deadlocks.

In addition under similar circumstances when the
bbp register couldn't be read the value must be
set to 0xff to indicate that the value is wrong.
This too didn't happen under all circumstances.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>