Bluetooth: hci_uart: Introduce h4_recv_buf helper function

The h4_recv_buf helper function can be used for receiving H:4 packets
out of a TTY stream. It is self-contained and allows for reuse by all
HCI UART protocols.

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