HSI: ssi_protocol: replace spin_lock with spin_lock_bh

To avoid setting irq_safe runtime pm flag in omap-ssi, multiple calls
will be moved to process context. This also affects ssi-protocol, so
use the safer spin_lock_bh instead of a simple spin_lock.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Tested-by: Pavel Machek <pavel@ucw.cz>