gpio: tps65218: use the new open drain callback

The TPS65218 supports open drain mode on its three pins,
with one of them configurable also as push-pull. Use the
new .set_single_ended() callback to set this up properly
from the core, so the core actually see it can drive the
pin(s) as open drain, and does not attempt to emulate
open drain by switching the pin to an input.

Acked-by: Nicolas Saenz Julienne <nicolassaenzj@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>