ALSA: hda - Add probe_mask default for Toshiba laptop with ALC268

Some Toshiba laptops have another ALC268 codec on slot#3 that conflicts
with the primary codec.  The codec#3 is for the digital I/O, and should
be fixed by the driver, but it'd need a bunch of changes.

So, let's fix the probe problem temporarily by setting the default
probe_mask value.

Reference: kernel bugzilla #12735
	http://bugzilla.kernel.org/show_bug.cgi?id=12735

Tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>