fsi: sbefifo: Add missing mutex_unlock

There was no unlock of the FFDC mutex.

Fixes: 9f4a8a2d ("fsi/sbefifo: Add driver for the SBE FIFO")
Signed-off-by: Eddie James <eajames@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>