usb: gadget: f_fs: signedness bug in __ffs_func_bind_do_descs()

We need "idx" to be signed for the error handling to work.

Fixes: 6d5c1c77 ('usb: gadget: f_fs: fix the redundant ep files problem')
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>