usb: gadget: atmel_usba: Cache INT_ENB register value

Cache INT_ENB register value in order to avoid uncached iomem access, and
thus improve access time to INT_ENB value.

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