[ALSA] hda-codec - Make internal speaker work on Acer C20x tablets

The following patch creates a new 'Mono speaker' control in alsamixer
when the Realtek 'acer' model is used with hda_intel.  This is needed so
the internal mono speaker (when present) can be controlled.
This new control won't do anything in Acer laptops which are not fitted with
a mono speaker. Acer models which are known to have a mono speaker are the
C20x tablet series but there may be others.  I guess we could define a new
model specifically for Acers with mono speakers but this seems a bit silly
given that such a model will be identical to the normal 'acer' model except
for this added control.
This patch also adds the C20x tablets to the list of PCI ids associated with
the 'acer' model.  This means that owners of C20x machines will no longer
have to supply 'model=acer' when loading hda_intel.

Signed-off-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>