atmel-usba-udc: use gpio_is_valid() to check vbus_pin I/O line

This patch will convert to use gpio_is_valid() to check the vbus_pin
platform data. It will also default to -ENODEV if no vbus_pin is
defined.

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