drivers: input: it8xxx2: resolve runtime assertion failure

When the kso[17:16] lines are enabled and `CONFIG_ASSERT` is
configured, the following error occurs: "Input cannot be
enabled for 'Open Drain', 'Open Source' modes without Output."
To address this, this commit configures kso16 and kso17 as
gpio output before applying the pinctrl settings.

Tested with: no abnormal keyscan occurs during
initialization

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>
Signed-off-by: Ren Chen <Ren.Chen@ite.com.tw>