usb: gadget: f_phonet: Fix sparse warning

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

drivers/usb/gadget/f_phonet.c:692:21: warning: symbol 'phonet_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_phonet.c

@@ -689,7 +689,7 @@ static void pn_unbind(struct usb_configuration *c, struct usb_function *f)
 	usb_free_all_descriptors(f);
 }
 
-struct usb_function *phonet_alloc(struct usb_function_instance *fi)
+static struct usb_function *phonet_alloc(struct usb_function_instance *fi)
 {
 	struct f_phonet *fp;
 	struct f_phonet_opts *opts;