ALSA: hda - Fix digital mic on dell-m4-1 and dell-m4-3

Fix num_dmuxes initialization for dell-m4-1 and dell-m4-3 models
of IDT 92HD71bxx codec, which was wrongly set to zero.

Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>