mwl8k: move ->peer_id from mwl8k_vif to mwl8k_sta

For STA firmware, move the per-peer hardware station ID to the
driver-private part of struct ieee80211_sta, where it belongs.

(Since issuing a hardware station database maintenance command sleeps,
we can't hold a reference to the ieee80211_sta * across the command,
and since we won't know the station ID until after the command
completes, we need to re-lookup the sta when the command is done to
write the returned station ID back to its driver-private part.)

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>