In order to deal with the weird and wonderful ways of obtaining a uart clock
which a lot of the high-speed hacks for 16550-based chips seem to have, we need
to make uart_update_timeout() and uart_get_divisor() take the desired baud rate
as their arguments, instead of termios or divisor as before. The main reason for the
change to uart_get_divisor(), requiring a call to uart_get_baud_rate() before it, is
so that the drivers actually _have_ the baud rate in order that they can pass it to
uart_update_timeout().