Bluetooth: hci_uart: Add new line discipline enhancements

Added the ability to flow control the UART, improved the UART baud
rate setting, transferred the speeds into line discipline from the
protocol and introduced the tty init function.

Signed-off-by: Ilya Faenson <ifaenson@broadcom.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>