ARM: at91: use -EINVAL for invalid gpio on atmel_mci

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>