USB: gadget: f_fs: use usb_string_ids_n()

Use usb_string_ids_n() function to simplify string ids
registeration.

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>