Convert old mass_storage gadget to use the new interface of f_mass_storage
so that later the compatibility layer in f_mass_storage can be removed.

struct fsg_common is not known to mass_storage.c, so a setter method
is added to f_mass_storage.
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>