serial: sh-sci: Fix up modem control handling.

At the moment things like CTS/RTS are reported for all ports, while the
vast majority of them do not implement support at all (and others
implement support entirely in hardware). Fix up the ->get_mctrl()
reporting to simply assert DSR/CAR as other drivers without control
lines do.

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