iwlwifi: mvm: use proper CDB check in PHY context modify

When the firmware supports CDB, PHY contexts cannot be modified to
change their band, but need to be added/remove instead. Instead of
relying on iwl_mvm_has_new_tx_api(), check the right FW capa flag
IWL_UCODE_TLV_CAPA_BINDING_CDB_SUPPORT and remove the comment.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>