ath5k: use ath5k_softc as driver data

It's our "private driver data"... It's used more often and hw is the mac80211
part. This makes more sense with the next (sysfs) patch.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>