serial: mctrl_gpio: add modem control read routine

mctrl_gpio_get_outputs() returns the state of following signals:

RTS, DTR

While defining the routine for reading outputs, fix the comment for
mctrl_gpio_get() routine as it returns only the state of the input
signals.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>