iwlwifi: mvm: fix build w/o CONFIG_THERMAL

Without CONFIG_THERMAL, the driver fails to link as it calls
iwl_mvm_send_temp_report_ths_cmd() unconditionally. Fix this
by making that function available, but do almost nothing but
send the empty firmware command to enable CT-kill reporting.

While at it, also fix that function itself to not error out
when the thermal zone hasn't been initialized, but instead
just send the empty firmware command in this case as well.

Fixes: 242d9c8b ("iwlwifi: mvm: use FW thermal monitoring regardless of CONFIG_THERMAL")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>