USB: serial: cp210x: make line-speed quantisation data driven

Older cp210x devices only support a fixed set of line speeds to which a
requested speed is mapped. Reimplement this mapping using a table
instead of a long if-else construct.

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