usb: gadget: udc: atmel: set vbus irqflags explicitly

The driver triggers actions on both edges of the vbus signal.

The former PIO controller was triggering IRQs on both falling and rising edges
by default. Newer PIO controller don't, so it's better to set it explicitly to
IRQF_TRIGGER_FALLING | IRQF_TRIGGER_RISING.

Without this patch we may trigger the connection with host but only on some
bouncing signal conditions and thus lose connecting events.

Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: stable <stable@vger.kernel.org> # v4.4+
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>