Currently, uart_get_divisor() and uart_get_baud_rate() take a tty
structure.  We really want them to take a termios structure so we
can avoid passing a tty structure all  the way down to the low level
drivers.

In order to do this, we need to be able to convert a termios
structure to a numeric baud rate - we provide tty_termios_baud_rate() in
tty_io.c for this purpose.  It performs a subset of the
tty_get_baud_rate() functionality, but without any "alt_speed"
kludge.

We finally export uart_get_baud_rate() and uart_get_divisor() to for
low level drivers to use.  We now have all the functions in place
to support ports which want to have access to the real baud rate
rather than a divisor value.