iwlwifi: mvm: don't read system time when modifying AP/GO MAC

When modifying a MAC, we update its beacon system time which
is taken as a base to calculate TBTT. The firmware doesn't use
the new timestamp because the time is never used after the MAC
and broadcast station were added, but it is safer to not rely
on this and avoids the overhead of reading the register every
time the MAC is updated.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>