mac80211: hardware should not deny going back to legacy

Doing so would be an MLME protocol violation when the peer disabled
the aggregation session. Quick driver review indicates that there are
error codes passed all over the drivers but cannot ever be nonzero
except in error conditions that would indicate mac80211 bugs.

No real changes here, since no drivers currently can return -EBUSY.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>