p54spi: Remove FIXME in op_stop

Don't use the interruptible variant of mutex_lock(). It doesn't really
need to be interruptible. This avoids nasty error handling.

Signed-off-by: Michael Buesch <m@bues.ch>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>