usb: gadget: mv_udc: fix hang when shutdown

Fix system hang in udc shutdown routine which caused by accessing usb
register when clock is disabled. So enable usb clock before access
register.

Signed-off-by: Yunfan Zhang <yfzhang@marvell.com>
Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>