ath9k: remove unnecessary debugfs return code checks

Since the ath9k debugfs directory is cleaned up by debugfs_remove_recursive,
there's no point in checking the return code of every single debugfs create
line.

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