CHROMIUM: ath9k: Do not divide by zero cycle counter

CHROMIUM: ath9k: Do not divide by zero cycle counter

cc_rxpoll.cycles can be zero under certain circumstances.
Skip this round of rx hang detection when this happens.

Signed-off-by: Paul Stewart <pstew@chromium.org>
BUG=chromium-os:32956
TEST=Reboot system.  Still waiting to see this message.

Change-Id: Id0a6d4b01137af4c36a27424615c30c52ed05fd8
Reviewed-on: https://gerrit.chromium.org/gerrit/28642
Reviewed-by: Simon Que <sque@chromium.org>
Reviewed-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>