iwlwifi: mvm: disallow new TDLS stations when appropriate

HW/FW constraints dictate that TDLS should only be used when a single
phy ctx is active. We also support at most 4 TDLS peers. We don't
support TDLS on a P2P vif.

Unify and move a phy-ctx counting implementation from the power-mgmt code
in order to simplify implementation.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>