ath9k: Fix regression in LNA diversity

The commit "ath9k: Optimize LNA check" tried
to use the "rs_firstaggr" flag to optimize the LNA
combining algorithm when processing subframes in
an A-MPDU. This doesn't appear to work well in practice,
so revert it and use the old method of relying on
"rs_moreaggr".

Cc: stable@vger.kernel.org # 3.11
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>