usb: gadget: f_ecm: fix missing unlock on error in ecm_alloc()

Add the missing unlock before return from function ecm_alloc()
in the error handling case.

Introduced by commit da92801c.
(usb: gadget: f_ecm: add configfs support)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/f_ecm.c

@@ -1012,6 +1012,7 @@ struct usb_function *ecm_alloc(struct usb_function_instance *fi)
 					  sizeof(ecm->ethaddr));
 	if (status < 12) {
 		kfree(ecm);
+		mutex_unlock(&opts->lock);
 		return ERR_PTR(-EINVAL);
 	}
 	ecm_string_defs[1].s = ecm->ethaddr;