wil6210: AP: prevent connecting to already connected station

wmi_evt_connect doesn't check if the connect event is received for
an already connected station.
This can lead to memory leak as a new vring is allocated without
freeing the previously allocated vring and to unexpected behavior
of nl80211 layer due to unexpected notification of a new station.

Add a check in wmi_evt_connect in AP mode to verify that the requested
CID is not associated to an already connected station.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>