USB: serial: ftdi_sio: clean up baudrate request

Multi-channel devices need to encode the channel selector in their
control requests and newer single-channel chip types use the same
request format.

Set the channel index also for these single-channel types so that the
index can be used to determine the baudrate request format instead of
listing types explicitly.

Note that FT232H and FTX accept either 0 or 1 as selector for their
single channel, presumably for backward compatibility reasons.

Signed-off-by: Johan Hovold <johan@kernel.org>