au6601: do some cleaning

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
au6601: make debug optional

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
au6601: more cleaning

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>