iwlwifi: mvm: don't set AP STA to EINVAL

Now that mac80211 calls pre_rcu_remove and we set the
fw_id_to_mac_id pointer to -ENOENT before the station is
removed, we don't need to set fw_id_to_mac_id to -EINVAL
when the station is really removed.
Leave fw_id_to_mac_id to be -ENOENT which will let the
drain worker (iwl_mvm_sta_drained_wk) know that this station
is not to be drained.
We don't need to drain this station since it is our AP in
managed mode and we flush all its frames synchronously
anyway.

Setting the AP station to -EINVAL could lead to confusion
since internal stations are also reserved with -EINVAL,
this confusion showed up in the logs as:

Drained sta 0, but it is internal?

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>