cp2101: Remove broken termios optimisation, use proper speed API

I've also enabled the commented out support for 7200, 14400, 55854,
127117 and 3686400 baud as you can now set such rates in the kernel.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>