ath9k: create a common debugfs_root for all device instances

The driver are trying to create an 'ath9k' directory in debugfs for each
device currently. If there are more than one device in the system, the
second try will always fail.

Changes-licensed-under: ISC

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>