USB: gadget: f_mass_storage: fsg_add() renamed to fsg_bind_config()

Mass Storage Function had a bit unique name for function
used to add it to USB configuration.  Renamed as to match
naming convention of other functions.

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