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

Convert the legacy multi gadget to the new 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>