USB: serial: ch341: add min and max line-speed macros

The line-speed algorithm clamps the requested value to the supported
range instead of bailing out on unsupported values.

Provide min and max macros and indicate how they are derived instead of
hardcoding the limits.

Note that the algorithm depends on the minimum rate (45.78 bps)
being rounded up (and the maximum rate being rounded down) to avoid
special casing.

Suggested-by: Michael Hanselmann <public@hansmi.ch>
Link: https://lore.kernel.org/r/20200630095756.GZ3334@localhost


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