ath9k: separate debugfs support from CONFIG_ATH_DEBUG

In my setups, ath9k's debugfs files are most of the time much more
useful than the messages generated by enabling CONFIG_ATH_DEBUG along
with the right debug flags.
Since CONFIG_ATH_DEBUG comes with a noticeable overhead on embedded
systems, this patch makes it possible to use the debugfs files without
that option.

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