Input: matrix_keypad - use flush_delayed_work()

We should be using flush_delayed_work() instead of flush_work() in
matrix_keypad_stop() to ensure that we are not missing work that is
scheduled but not yet put in the workqueue (i.e. its delay timer has not
expired yet).

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>