serial: sh-sci: per-port modem control.

The bulk of the ports do not support any sort of modem control, so
blindly twiddling the MCE bit doesn't accomplish much. We now require
ports to manually specify which line supports modem control signals.

While at it, tidy up the RTS/CTSIO handling in SCSPTR parts so it's a bit
more obvious what's going on (and without clobbering other configurations
in the process).

Signed-off-by: Paul Mundt <lethal@linux-sh.org>