USB: gadget/at91_udc: add gpio_to_irq() function to vbus interrupt

Now that we are using irqdomains, we need to convert GPIO pins to Linux
IRQ numbers using the gpio_to_irq() function.
This call is added to request/free_irq calls.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Cc: linux-usb@vger.kernel.org