ALSA: hda - Clean up the code to check bdl_pos_adj option

Just a minor cleanup; instead of passing an array, pass the assigned
bdl_pos_adj option value directory in struct azx.  Also split the code
to get the default bdl_pos_adj value for the change that will follow
after this.

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