USB: cypress_m8: Limit baud rate to <=4800 for USB low speed devices

The cypress app note for the M8 states that for the USB low speed
version of the part, throughput is effectively limited to 800
bytes/sec.  So if we were to try a faster baud rate in such cases then
we risk overrun errors on receive.  Best to just identify this case
and limit the rate to 4800 baud or less (by ignoring any request to
set a faster rate).  The old baud rate setting code was somewhat
fragile; this change also hopefully makes it easier in the future to
better checking / limiting.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>