CHROMIUM: atkbd: improve workaround for chromeos EC bug

CHROMIUM: atkbd: improve workaround for chromeos EC bug

This makes the workaround for commit b483f9730b97ba8812340cf1584b491d5aa1e774
(https://gerrit.chromium.org/gerrit/43049, "CHROMIUM: atkbd: workaround
for ChromeOS EC keyboard enable bug") work properly by re-enabling the
keyboard IRQ which was accidentally disabled.

BUG=chrome-os-partner:17810
TEST=tested with > 200 power cycles and > 30 workaround triggers
BRANCH=none

Change-Id: Ia8a817f665a3575eebeb1ba2ec3aa89f80c3e0e3
Signed-off-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43300