ath9k: fix roadkill due to Joe's patch on ath_dbg() changes

Joe changed ath_dbg() to simpify code but while his patch was
being merged dfs.c was born and as such did not get the change
Joe envisioned. This fixes that. Test compiled with:

 make allmodconfig

Cc: Joe Perches <joe@perches.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: John W. Linville <linville@tuxdriver.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>