ARM: at91: fix check of valid GPIO for SPI and USB

SPI chip select pins have to be checked by gpio_is_valid().
The USB host overcurrent_pin checking was missing.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>