ath9k: Optimize LNA check

The documentation for antenna diversity says:

"The decision of diversity is done at 802.11 preamble. So, for
11G/11B, for every MAC packet hardware will do a decision. But in
11N with aggregation, the decision is made only at the preamble and
all other MPDUs will use the same LNA as the first MPDU."

Make use of rs_firstaggr to avoid needlessly scanning for LNA
changes.

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