ath5k: Move debugfs under ieee80211/[wiphy-name]

This automatically keeps things proper when wiphy
is renamed.

Based on patch by Johannes Berg <johannes@sipsolutions.net>

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