ath9k: Fix max AMPDU size calculation

The maximum A-MPDU size is calculated in ath_tx_aggr_start(),
so there is no need to do it in node_attach() too. Also, make
sure that the correct size is calculated as described in
8.4.2.58.3.

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