ath6kl: convert ath6kl_dbg() and ath6kl_dbg_dump() into functions

That way it's possible to not export debug_mask outside the upcoming
ath6kl_core.ko and that makes it easier to ath6kl_core.ko in the
following patch.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>