Input: tca8418_keypad - remove double read of key event register

There is no need to tread the same register twice in a row.

Fixes: ea4348c8 ("Input: tca8418_keypad - hide gcc-4.9 -Wmaybe-un ...")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>