Unregister all netdevs before freeing local data. I was unable to
trigger any crashes without this change when running busy loops for
driver operations when ejecting a Prism2 PC Card. Anyway, should there
be a race condition with this, better make it less likely to happen by
unregistering the netdevs first.

Signed-off-by: Jouni Malinen <jkmaline@cc.hut.fi>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>