USB: gadgets: ci13xxx: fix probing of compiled-in gadget drivers

Built-in gadget drivers have NULL-ifed unbind() function. Checking
whether unbind() is NULL will never let any compiled into kernel
driver attach.

Signed-off-by: Artem Leonenko <tikkeri@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/ci13xxx_udc.c

@@ -2393,7 +2393,6 @@ int usb_gadget_probe_driver(struct usb_gadget_driver *driver,
 
 	if (driver             == NULL ||
 	    bind               == NULL ||
-	    driver->unbind     == NULL ||
 	    driver->setup      == NULL ||
 	    driver->disconnect == NULL ||
 	    driver->suspend    == NULL ||