usb: gadget: acm_ms: convert to new interface of f_mass_storage

Convert the legacy acm_ms gadget to use the new function interface
of f_mass_storage, so that later the compatibility layer in
f_mass_storage can be removed.

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>