input: Monitor both onkey up and down event

Remove original 3-second ONKEY event. Detect ONKEY changing event directly.
So both UP and DOWN event of ONKEY in max8925 are monitered.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>