USB: f81232: clarify f81232_ioctl() and fix

We extract TIOCGSERIAL section in f81232_ioctl() to f81232_get_serial_info()
to make it clarify.

Also we fix device type from 16654 to 16550A, and set it's baud_base
to 115200 (1.8432MHz/16).

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