We provide a new function, uart_get_baud_rate(), to return the
desired numeric baud rate from the uart_port and tty structures.
This allows us to:

1. localise the 38400 alternate speed kludge to one area.
2. support faster baud rates than the usual baud_base / desired_rate
   calculations in the future (eventually allowing the use of the
   magic divisors in 8250.c.)