orinoco: convert scanning to cfg80211

This removes the custom scan cache used by orinoco.

We also have to avoid calling cfg80211_scan_done from the hard
interrupt, so we offload the entirety of scan processing to a workqueue.

This may behave strangely if you start scanning just prior to
suspending...

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>