iwlwifi: mvm: better handle several several vifs in BT Coex

When there one vif on 5GHz associating, it would clear all
the BT Coex constraints. This can't work if there is
another vif on 2.4GHz. Fix that.

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