ALSA: hda: fixed hp_nid DAC for DELL_M6

This patch sets the HP out not used by the "Headphone to Line Out" switch to the
hp_nid.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>