ath9k: Remove RX Poll

This patch removes the convoluted and hacky method of
monitoring for connectivity. We rely on mac80211's connection
loss logic and doing it in the driver is not necessary.

The HW check for MAC/BB hangs is also simplified, there
is no need to have a separate work instance for it.

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