ath5k: enable Power-Save Polls by setting the association ID

mac80211 has long provided us the association ID. This isn't useful except
for Power-Save polling which now gets enabled. We can now poll for our
pending frames on the AP during power save.

You can review the details of Power-Save on the wireless wiki:

http://wireless.kernel.org/en/developers/Documentation/ieee80211/power-savings

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>