ALSA: HDA: Fix internal microphone on Dell Studio 16 XPS 1645

More than one user reports that changing the model from "both" to
"dmic" makes their Internal Mic work.

Cc: stable@kernel.org
Tested-by: Martin Ling <martin-launchpad@earth.li>
BugLink: https://bugs.launchpad.net/bugs/795823


Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>