usb: gadget: mass_storage: merge usb_f_mass_storage module with u_ms module

u_ms.ko is needed only together with usb_f_mass_storage.ko. Merge them.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>