usb: gadget: atmel_usba_udc: Fixed vbus_prev initial state

If vbus gpio is high at init, we should set vbus_prev to true
accordingly to the current vbus state. Without that, we skip the first
vbus interrupt because the saved vbus state is not consistent.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>