Input: gpio-keys - avoid possibility of sleeping in timer function

The gpio_get_value function may sleep, so it should not be called in a
timer function. Move gpio_get_value calls to workqueue.

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