ath9k: Send legacy rated frames as unaggregated

Currently the aggregation is formed till the aggregation limit
is reached and the rate lookup is done for the first frame alone.
But there can be a legacy rated frames in tid queue. This patch
limits the subframe addition based on presence of legacy rate and
sends the legacy rated frames as unaggregated one.

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