serial: mctrl_gpio: Use gpiod flags directly

Description of the modem line control GPIOs contain a boolean type to set
direction of the line. Since GPIO library provides an enumerator type of flags,
we may utilize it and allow a bit more flexibility on the choice of the type of
the line parameters. It also removes an additional layer of value conversion.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190814140759.17486-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>