usb: Return -ENOSYS when system call is not available

Update usb_gadget_release() to use -ENOSYS, which is the correct error
code for U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/gadget/udc/udc-uclass.c b/drivers/usb/gadget/udc/udc-uclass.c
index 3053ccf..dbc354e 100644
--- a/drivers/usb/gadget/udc/udc-uclass.c
+++ b/drivers/usb/gadget/udc/udc-uclass.c
@@ -45,7 +45,7 @@
 		dev_array[index] = NULL;
 	return ret;
 #else
-	return -ENOTSUPP;
+	return -ENOSYS;
 #endif
 }