ALSA: hda - Clean up multi-channel mixer name assignment in patch_realtek.c

Change alc_get_line_out_pfx() in patch_realtek.c to provide the channel
specific name and assign the index so that each caller doesn't have to
set the channel name by itself.

Also, check the multi-io case with the primary hp-out; for the multi-io
channels, assign the channel name instead of "Headphone" with indices.
This makes the mixer names more intuitive and reduces confusion.

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