iwlwifi: don't declare support for 5ghz if not supported

Remove a useless debug print about unsupported channels.
Also add a comment about the LAR special case where channels
might become valid later.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>