usb: atmel_usba_udc: Mask status with enabled irqs

Avoid interpreting useless status flags when we're not waiting for such
events by masking the status variable with the interrupt enabled register
value.

Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Reported-by: Patrice VILCHEZ <patrice.vilchez@atmel.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>