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.

Change-Id: I0e438d437154b1b47c572218072a736da5013c80
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Git-commit: 3d287fb398c03189a1394778162f6404e4d44ad2
Git-repo: https://github.com/kvalo/ath.git


CRs-Fixed: 982882
Signed-off-by: Maya Erez <merez@codeaurora.org>