serial: two branches the same in timbuart_set_mctrl()

CTS is a read only bit and we are to stop signal RTS if modem line
TIOCM_RTS is not set.

Thanks for suggestions by Richard Röjfors.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Richard Röjfors <richard.rojfors@pelagicore.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>