ALSA: hda: make a STAC_DELL_EQ option

Add support for explicitly enabling the EQ distortion hack for
systems without software biquad support.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>