gpio/omap: disable debounce on idle

After omap2_gpio_prepare_for_idle call it's not guaranteed that given
GPIO module will hit idle, so it might end up in active state but with
debounce clock disabled, which will break input functionality.

Disable debounce feature on idle to avoid this problem.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>