USB: f81232: implement set_termios()

The original driver had do not any h/w change in driver.
This patch implements with configure H/W for
baud/parity/word length/stop bits functional in f81232_set_termios().

This patch also implement DTR/RTS control when baudrate B0.
We drop DTR/RTS when B0, otherwise enable it.

We are checking baudrate in set_termios() too, If baudrate larger then 115200,
it will be changed to 115200 and use tty_encode_baud_rate() to encode into tty

Signed-off-by: Peter Hung <hpeter+linux_kernel@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>