ath9k: fix compile error without debug enabled

commits 8e6f5aa2 and
db1a052b accidentally introduced
compile errors that happens when ath9k debug is not enabled.
This patch fixes the declaration of the inline stubs to resolve this.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>