ARM: at91/usbh: fix overcurrent gpio setup

Use gpio_is_valid also for overcurrent pins (which are currently
negative in many board files).

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