mac80211: properly deal with station hashtable insert errors

mac80211: properly deal with station hashtable insert errors

The original hand-implemented hash-table in mac80211 couldn't result
in insertion errors, and while converting to rhashtable I evidently
forgot to check the errors.

This surfaced now only because Ben is adding many identical keys and
that resulted in hidden insertion errors.

Cc: [email protected]
Fixes: 7bedd0cfad4e1 ("mac80211: use rhashtable for station table")
Reported-by: Ben Greear <[email protected]>
Signed-off-by: Johannes Berg <[email protected]>