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

commit ffe535ed upstream.

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>