wlcore: set max_rx_agg_subframes in mac80211 according to HT conf

The max Rx aggregation subframes configured to FW must be the same
number given to the upper layers (mac80211). Derive both from the same
value, given in the conf struct.

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