ALSA: hda/realtek - Add a fixup entry for Acer Aspire 8940G

It's compatible with 8930G.
Using the same fixup gives the proper 5.1 sound back.

Reported-and-tested-by: Dany Martineau <dany.luc.martineau@gmail.com>
Cc: <stable@kernel.org> [v3.3+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>