serial: mctrl_gpio: use gpiod_set_array function

Make the serial_mctrl_gpio driver the first user of the new gpiod_set_array
function, which is now available in the linux-gpio devel tree.
All modem control output signals are now set simultaneously.

Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>