p54spi: fix potential null deref in p54spi.c

Fix a potential NULL dereference bug during
error handling in p54spi_probe.

This bug was discovered by smatch:
(http://repo.or.cz/w/smatch.git).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>