ath9k: Move debugfs under ieee80211/[phyname]/ath9k/

This fixes debugfs problems when a phy is renamed,
and is able to remove a bit of code that is no longer
needed.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>