usb: gadget: mv_udc: fix the warning of mv_udc_remove

The __exit_p() will be NULL if MODULE is no defined.
It will cause the warning. Removing __exit_p to remove
the warning.

Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>