iwlwifi: mvm: don't ask beacons when P2P GO vif and no assoc sta

The commit below called iwl_mvm_mac_ctxt_changed() to handle
a case that the vif is a P2P GO.
However iwl_mvm_mac_ctxt_cmd_go() ignores the number of
associated stations and asks the FW to pass beacons anyways.

Fix this by checking ap_assoc_sta_count parameter, in iwl_mvm_mac_ctxt_cmd_go()
as well, and ask the FW to pass beacons only when there's
at least one associated station.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>