ath9k: Fix MAC HW hang check for AR9003

The current method of identifying MAC hangs is
convoluted and also, the signatures are wrong and
don't apply to all the chips in the AR9003 family.
Fix this by cleaning up the code and checking for
the correct hang signatures.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>