usb: gadget: f_serial: Fix sparse warning

Make local symbol static in order to fix the following sparse
warning.

drivers/usb/gadget/f_serial.c:357:21: warning: symbol 'gser_alloc' was not declared. Should it be static?
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_serial.c

@@ -354,7 +354,7 @@ static void gser_unbind(struct usb_configuration *c, struct usb_function *f)
 	usb_free_all_descriptors(f);
 }
 
-struct usb_function *gser_alloc(struct usb_function_instance *fi)
+static struct usb_function *gser_alloc(struct usb_function_instance *fi)
 {
 	struct f_gser	*gser;
 	struct f_serial_opts *opts;