qcacmn: dfs_set_cfreq2 may be set wrongly when start SAP on 2G channel

dfs_curchan isn't reset when start vdev on 2G channel, can't use it.
Reproduce steps:
1. DUT vdev1 SAP started on 5G DFS channel with HE160,
WMI_CHAN_FLAG_DFS_CFREQ2 is set.
2. DUT vdev1 SAP switched to 2G channel.
Found WMI_CHAN_FLAG_DFS_CFREQ2 still set.

Change-Id: Ic5282ddf667af92365f3e43274a010a26a5a0715
CRs-Fixed: 2787305