ALSA: intel8x0: Mute External Amplifier by default for ThinkPad X31

BugLink: https://bugs.launchpad.net/bugs/619439



This ThinkPad model needs External Amplifier muted for audible playback,
so set the inv_eapd quirk for it.

Reported-and-tested-by: Dennis Bell <dennis.bell@parkerg.co.uk>
Cc: <stable@kernel.org>
Signed-off-by: Daniel T Chen <crimsun@ubuntu.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>