Bluetooth: hci_uart: Remove unused h4->rx_state and h4->rx_count field

The fields h4->rx_state and h4->rx_count are not used at all and with
that they can be just removed.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>