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>

drivers/usb/gadget/function/f_fs.c

@@ -2352,7 +2352,8 @@ static int __ffs_func_bind_do_descs(enum ffs_entity_type type, u8 *valuep,
 	struct usb_endpoint_descriptor *ds = (void *)desc;
 	struct ffs_function *func = priv;
 	struct ffs_ep *ffs_ep;
-	unsigned ep_desc_id, idx;
+	unsigned ep_desc_id;
+	int idx;
 	static const char *speed_names[] = { "full", "high", "super" };
 
 	if (type != FFS_DESCRIPTOR)