usb: gadget: tcm_usb_gadget: mark bot_cleanup_old_alt static

that symbol is only used inside this function driver,
we should mark it static. Fixes sparse's:

drivers/usb/gadget/tcm_usb_gadget.c:373:6: warning: symbol \
	'bot_cleanup_old_alt' was not declared. Should it \
	be static?

Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/tcm_usb_gadget.c

@@ -370,7 +370,7 @@ err:
 	return -ENOMEM;
 }
 
-void bot_cleanup_old_alt(struct f_uas *fu)
+static void bot_cleanup_old_alt(struct f_uas *fu)
 {
 	if (!(fu->flags & USBG_ENABLED))
 		return;