iwlwifi: mvm: modify the tsf_id master/slave logic

For TSF master/slave synchronization, the FW does not require
exact match in the beacon interval between the master interface
and the slave one, but instead requires that the beacon interval
of one interface is the module of the other.

Modify the tsf_id selection to align with the above.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>