tty: Re-add external interface for tty_set_termios()

This is needed by Bluetooth hci_uart module to be able to change speed
of Bluetooth controller and local UART.

Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>