iwlwifi: mvm: prevent multiple stations with the same address

As the device (and parts of the driver) cannot deal with having the
same MAC address for two stations (on two virtual interfaces), add
some explicit code to prevent this case.

Note that in practice this cannot happen since the device doesn't
support operating with two AP/GO interfaces at the same time either,
and other scenarios for this are, while not impossible, not going to
happen in practice.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>