ath9k_htc: Fix max subframe handling

Commit "ath9k_htc: Fix AMPDU subframe handling" registered the maximum
subframe limit of the driver with mac80211, which was used in ADDBA
negotiation. While technically correct, this causes inter-operability issues
with a few APs. Revert to the older behavior to fix this.

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