[ALSA] hda-codec - Fix ALC882 capture source selection

The capture source selection for ADC list with two elements is buggy
becaues of a wrong capture mux list.  This patch fixes the starting
index based on spec->num_adc_nids.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>