mac80211: use sdata pointer for scan interface

Since we now use sdata pointers most of the time, using a netdev
pointer here is somewhat artificial, use an sdata pointer instead.
Replace a netdev-prefix in a few messages by a wiphy-prefix.

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