usb: gadget: f_mass_storage: Fix sparse warning

Use NULL instead of 0 when returning pointer, to fix the following
sparse warnings.

drivers/usb/gadget/f_mass_storage.c:3114:60: warning: Using plain integer as NULL pointer
drivers/usb/gadget/f_mass_storage.c:3114:63: warning: Using plain integer as NULL pointer
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_mass_storage.c

@@ -3111,7 +3111,7 @@ static int fsg_bind(struct usb_configuration *c, struct usb_function *f)
 					  fsg->common->can_stall);
 		if (ret)
 			return ret;
-		fsg_common_set_inquiry_string(fsg->common, 0, 0);
+		fsg_common_set_inquiry_string(fsg->common, NULL, NULL);
 		ret = fsg_common_run_thread(fsg->common);
 		if (ret)
 			return ret;