USB: g_mass_storage: Mass Storage Function created

The f_mass_storage.c has been changed into a composite function.
mass_storage.c file has been introduced which defines a
g_mass_storage gadget based on composite framework.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>