omap: GPIO: fix auto-disable of debounce clock

The addition of the new debounce code (commit
168ef3d9) broke the auto-disable of
debounce clocks on idle by forgetting to update the debounce clock
enable mask.

Add back the updating of bank->dbck_enable_mask so debounce clocks are
auto-disabled.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>