ALSA: hda - Use a macro for snd_array iteration loops

Introduce a new helper macro, snd_array_for_each(), to iterate for
each snd_array element.  It slightly improves the readability than
lengthy open codes at each place.

Along with it, add const prefix to some obvious places.

There should be no functional changes by this.

Signed-off-by: Takashi Iwai <tiwai@suse.de>