ALSA: hda - Don't check capture source mixer if no ADC is available

With multiple codec configurations, some codec might have no ADC, thus
it keeps spec->adc_nids = NULL.  This causes an Oops in alc_build_controls().

Reference: kernel bug #16156
	https://bugzilla.kernel.org/show_bug.cgi?id=16156

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

sound/pci/hda/patch_realtek.c

@@ -2619,6 +2619,7 @@ static int alc_build_controls(struct hda_codec *codec)
 	}
 
 	/* assign Capture Source enums to NID */
+	if (spec->capsrc_nids || spec->adc_nids) {
 		kctl = snd_hda_find_mixer_ctl(codec, "Capture Source");
 		if (!kctl)
 			kctl = snd_hda_find_mixer_ctl(codec, "Input Source");
@@ -2630,6 +2631,7 @@ static int alc_build_controls(struct hda_codec *codec)
 			if (err < 0)
 				return err;
 		}
+	}
 	if (spec->cap_mixer) {
 		const char *kname = kctl ? kctl->id.name : NULL;
 		for (knew = spec->cap_mixer; knew->name; knew++) {