usb: gadget: Compilation warning fix

A compilation warning was added by the patch
"usb: gadget: use config_ep_by_speed() instead of ep_choose()".
This patch fixed it.
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/storage_common.c

@@ -494,7 +494,7 @@ static struct usb_descriptor_header *fsg_hs_function[] = {
 };
 
 /* Maxpacket and other transfer characteristics vary by speed. */
-static struct usb_endpoint_descriptor *
+static __maybe_unused struct usb_endpoint_descriptor *
 fsg_ep_desc(struct usb_gadget *g, struct usb_endpoint_descriptor *fs,
 		struct usb_endpoint_descriptor *hs)
 {