staging: rtl8712: return -ENOMEM instead of -1

After the commit 91d435fe replaced _malloc with kmalloc, smatch picked up a
couple of new warnings. This fixes the warning:

warn: returning -1 instead of -ENOMEM is sloppy
Signed-off-by: Vitaly Osipov <vitaly.osipov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/usb_ops_linux.c

@@ -499,7 +499,7 @@ int r8712_usbctrl_vendorreq(struct intf_priv *pintfpriv, u8 request, u16 value,
 	if (palloc_buf == NULL) {
 		dev_err(&udev->dev, "%s: Can't alloc memory for vendor request\n",
 			__func__);
-		return -1;
+		return -ENOMEM;
 	}
 	pIo_buf = palloc_buf + 16 - ((addr_t)(palloc_buf) & 0x0f);
 	if (requesttype == 0x01) {