Bluetooth: btnxpuart: remove useless assignment

Remove useless assignment of rx_skb to NULL in case the skb is in error,
this is already done in h4_recv_buf() that is executed a few lines
before.
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

drivers/bluetooth/btnxpuart.c

@@ -1278,7 +1278,6 @@ static int btnxpuart_receive_buf(struct serdev_device *serdev, const u8 *data,
 		/* Safe to ignore out-of-sync bootloader signatures */
 		if (!is_fw_downloading(nxpdev))
 			bt_dev_err(nxpdev->hdev, "Frame reassembly failed (%d)", err);
-		nxpdev->rx_skb = NULL;
 		return count;
 	}
 	if (!is_fw_downloading(nxpdev))