ALSA: intel8x0: Mute External Amplifier by default for Gateway 4525GZ

BugLink: https://bugs.launchpad.net/bugs/487884



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

Signed-off-by: Daniel T Chen <crimsun@ubuntu.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>