p54spi: Add missing spin_lock_init

The tx_lock is not initialized properly. Add spin_lock_init().

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