usb: gadget: f_subset: fix missing unlock on error in geth_alloc()

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

Introduced by commit 02832e56.
(usb: gadget: f_subset: 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_subset.c

@@ -548,6 +548,7 @@ static struct usb_function *geth_alloc(struct usb_function_instance *fi)
 					  sizeof(geth->ethaddr));
 	if (status < 12) {
 		kfree(geth);
+		mutex_unlock(&opts->lock);
 		return ERR_PTR(-EINVAL);
 	}
 	geth_string_defs[1].s = geth->ethaddr;