Input: gpio-keys - revert 'change timer to workqueue'

This reverts commit 0b346838c5862bfe911432956a106d602535d030.

This commit breaks GPIO debouncing by replacing the original mod_timer
with schedule_delayed_work in the interrupt handler. The latter does not
kick the timer further on GPIO line changes as it should to perform
debouncing.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>