ath6kl: Max clients reached notification

When a station requests connection to an AP, that has already been
connected to the maximum number of stations it can support, an event
is sent to user space via NL80211_CMD_CONN_FAILED command and reason
attribute NL80211_ATTR_CONN_FAILED_REASON with
NL80211_CONN_FAIL_MAX_CLIENTS as reason.

Signed-off-by: Pandiyarajan Pitchaimuthu <c_ppitch@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>