ath6kl: Fix invalid pointer access on fuzz testing with AP mode

ath6kl: Fix invalid pointer access on fuzz testing with AP mode

In our Fuz testing, reference client corrupts the dest mac to "00:00:00:00:00:00"
in the WPA2 handshake no 2. During driver init the sta_list entries mac
addresses are by default "00:00:00:00:00:00". Driver returns an invalid
pointer (conn) and the drver shall crash, if rxtids (aggr_conn)
skb queues are accessed, since they would not be initialized.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>