iwlwifi: mvm: don't set the MCAST queue in STA's queue list

The MCAST queue should be enabled after DTIM only.
According to fw API, the MCAST must not be attached to any
station, but should appear in the mcast_qid of the AP's
mac context only.

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