usb: gadget: g_ffs: convert to new interface of f_fs

Prepare for configfs integration. Use the new interface so that f_fs can be
made a module.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>