wifi: iwlwifi: mvm: send time sync only if needed

If there's no peer configured then there's no point in sending
the command down to the firmware with an invalid peer address.

Fixes: cf85123a

 ("wifi: iwlwifi: mvm: support enabling and disabling HW timestamping")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230531194630.0fb9f81f1852.Idcc41b67d1fbb421e5ed9bac2177b948b7b4d1c9@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>