USB: serial: cp210x: honour device-type maximum line speed

USB: serial: cp210x: honour device-type maximum line speed

Newer cp210x devices support higher line speeds than the older ones
which supported a discrete set of speeds up to 921.6 kbaud.

To support these higher speeds, we have for some time mapped speeds
lower than 1 Mbaud to the speeds supported by older devices, while
allowing the device to pick the closest possible rate for higher speeds
(without trying to guess and report back what rate was actually chosen).

As this implementation can lead to undefined behaviour for older devices
which do not support the higher rates, let's use the later-added
device-type detection to determine the maximum supported speed.

This will also be useful when adding support for cp2102n which can
handle rates up to 3 Mbaud.

As per the data sheets the following maximum speeds are used

	cp2101		921.6 kbaud
	cp2102/3	1 Mbaud
	cp2104/8	2 Mbaud
	cp2105
	 - ECI port	2 Mbaud
	 - SCI port	921.6 kbaud

while keeping the maximum 2 Mbaud for unknown device types in order to
avoid any regressions.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>