libertas: stop using ieee80211 for radiotap device

There seems to be no point in doing it as an ieee80211 device instead of
a normal netdev, and when we override its ->priv and then call
free_ieee80211() it has a distressing tendency to crash horribly.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>