Input: cros_ec_keyb: mask out extra flags in event_type

http://crosreview.com/1341159

 added a EC_MKBP_HAS_MORE_EVENTS flag to
the event_type field, the receiver side should mask out this extra bit when
processing the event.

Signed-off-by: Ting Shen <phoenixshen@chromium.org>
Reviewed-by: Enrico Granata <egranata@google.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>