avr32: use gpio_is_valid() to check USBA vbus_pin I/O line

This patch will use gpio_is_valid() to check the vbus_pin I/O line.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>