gpio: menz127: use the new open drain callback

The menz127 driver tries to support open drain by detecting it
at request time. However: without the new callbacks from the
gpiolib it is not really working: the core will still just emulate
the open drain mode by switching the line to an input.

By adding a hook into the new .set_single_ended() call rather than
trying to autodetect at request() time, proper open drain can be
supported.

Cc: Andreas Werner <andy@wernerandy.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>