qcacld-3.0: Fix the channel switch to 2.4Ghz after pre-cac

In case of pre-cac as part of pre-cac vdev delete the whole
mac_ctx->sap is zero in sap_clear_global_dfs_param() and thus the
loop for mac_ctx->sap.sapCtxList always return null sap ctx. Due
to this if radar is dedected on the original SAP, the vdev switch
to new channel is ignored.

Also in case of radar detection if SAP moves to 2.4ghz, the HW mode
change is not attempted and thus vdev restart fails on 2.4ghz if HW
mode is not DBS.

Thus dont reset the mac_ctx->sap in sap_clear_global_dfs_param() and
add check if HW mode change is required when channel change due to
Radar detection.

Change-Id: Id44a0ddbbb20035412b0c039c12f7f59f2c37f1a
CRs-Fixed: 2753029