qcacld-3.0: Ignore CAC if STA already present

Currently the driver sets the CAC required for
SAP if it changes it channel to DFS, it
issues a broadcast deauth to all clients,
which leads to unwanted disconnection, which
can be avoided if any STA if already connected
to an AP on that channel.
This is because there is already active TX going
on that channel, so the SAP need not wait for
CAC time to sense the medium.

Fix is to check whether any STA vdev operating
frequency matches with the target frequency of
the SAP, and if it is true, then do not set the
CAC required to true in SAP's vdev.

Change-Id: I09dbecc1a4625cb51e6095f7579479413e525a74
CRs-Fixed: 2643790