Input: cros_ec_keyb - add back missing mask for event_type

In the previous patch we didn't mask out event_type in case statement,
so switches are always picked instead of buttons, which results in
ChromeOS devices misbehaving when power button is pressed.
This patch adds back the missing mask.

Fixes: d096aa3e ("Input: cros_ec_keyb: mask out extra flags in event_type")
Signed-off-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>