wl12xx: set mac80211 flags for A-MPDU aggregation support

We set the mac80211 flag for A-MPDU support and also indicate that
Tx-agg session setup is performed in HW.

This patch depends on
"mac80211: add flag to indicate HW only Tx-agg setup support"

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>