iwlwifi: mvm: don't leak a station when we drain

We had a bug that prevented us from removing a station
after we entered the drain flow:

We assign sta to be NULL if it was an error value.
Then we tested it against -EBUSY, but forget to retrieve
the value again from mvm->fw_id_to_mac_id[sta_id].

Due to this bug, we ended up never removing the STA from
the firmware. This led to an firmware assert when we remove
the GO vif.

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