gpio: syscon: Fix possible NULL ptr usage

The priv->data->set can be NULL while flags contains GPIO_SYSCON_FEAT_OUT
and chip->set is valid pointer. This happens in case the controller uses
the default GPIO setter. Always use chip->set to access the setter to avoid
possible NULL pointer dereferencing.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>