usb: gadget: FunctionFS: staticize functions used only in f_fs.c

ffs_alloc_dev and ffs_free_dev are used only in f_fs.c,
so make them static.

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