ALSA: azt3328: improve snd_azf3328_codec_setdmaa()

- add some WARN_ONCE
- add multi-I/O helper (and use helper struct)
- fix off-by-1 DMA length bug
- better variable naming

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>