qcacmn: Avoid array out of bound access for current channels list

Currently regulatory computes the current channel list for band
6GHz by default which is not correct as current channel list
size is based on the CONFIG_BAND_6GHZ, if this feature is not
enabled then current channel list array will be accessed out of
bound inside reg modify channel list for band api.

To address above issue, move computation of current channel list
functionality inside CONFIG_BAND_6GHZ feature flag.

Change-Id: I4612c9d3adb027976a4404fb7c92c0e415014b47
CRs-Fixed: 2760541