serial: sh-sci: report CTS as active for get_mctrl

sh-sci.c sets hardware up and then let the HW do all flow controls.
There is no software code, nor needs to get/set real CTS signal.

But, when turning CRTSCTS on through termios, uart_set_termios() in
serial_core.c checks CTS, and stops TX if it is inactive at the moment.

Because sci_get_mctrl() returns a fixed value DTR|RTS|DSR but CTS,
the sequence
  open -> set CRTSCTS -> write
hit the case and stop working, no more outputs.

This patch makes sci_get_mctrl() report CTS in addition.

Signed-off-by: Takashi YOSHII <takashi.yoshii.zj@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>