ALSA: hda: Use STAC_DELL_M6_BOTH quirk for Dell Studio 1558

BugLink: https://launchpad.net/bugs/568600

The OR has verified that the dell-m6 model quirk is necessary for audio
to be audible by default on the Dell Studio XPS 1645.

This change is necessary for 2.6.32.11 and 2.6.33.2 alike.

Reported-by: Andy Ross <andy@plausible.org>
Tested-by: Andy Ross <andy@plausible.org>
Cc: <stable@kernel.org>
Signed-off-by: Daniel T Chen <crimsun@ubuntu.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>