USB: g_mass_storage: fixed module name in Kconfig

The Kconfig help message for Mass Storage Gadget claimed the
module will be named "g_file_storage" whereas it should be
"g_mass_storage".

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