Cast vendor-specific spell on VIA VT1818S codecs alike to VT1708S to

make analog input loopback and dual-stream playback work by enabling
signal mixing by nid 22, as it should be according to info returned by
the CODEC.  Otherwise pin nid 28 receives only signal from DAC nid 16.

PR:		kern/169124
MFC after:	1 week

sys/dev/sound/pci/hda/hdaa_patches.c

@@ -608,6 +608,8 @@ hdaa_patch_direct(struct hdaa_devinfo *devinfo)
 		/* Enable Mic Boost Volume controls. */
 		hda_command(dev, HDA_CMD_12BIT(0, devinfo->nid,
 		    0xf98, 0x01));
+		/* Fall though */
+	case HDA_CODEC_VT1818S:
 		/* Don't bypass mixer. */
 		hda_command(dev, HDA_CMD_12BIT(0, devinfo->nid,
 		    0xf88, 0xc0));