iwlwifi: mvm: report FTM start time TSF when applicable

When the interface that is requesting an FTM measurement is connected
to a BSS, it is possible that the FTM request was originated by an
RRM request from the AP. In this case the station needs to report
the measurement start time in terms of the TSF of the AP.

Since there is no indication in the FTM request itself if the TSF
is needed, always report the TSF if the station is associated.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>