atheros: add common debug printing

ath9k uses this for now, ath9k_htc is expected to re-use this
as well. We lave ath5k as is, but it certainly can also be
converted later.

The ath9k module parameter and debugfs entry is kept.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>