ALSA: usb: Fix wrong mapping of RLC and RRC channels

According to USB Audio spec v2 bits 25 and 26 of bmChannelConfig are
"Back Left of Center - BLC" and "Back Right of Center - BRC",
respectively.

They are currently assigned to ALSA channels BLC/BRC. However, the ALSA
BLC/BRC are actually the rather nonsensical "bottom left center" and
"bottom right center", so the channels will be assigned wrongly. The
comments in the USB code are also similarly wrong, so this is not
readily apparent without looking at the actual specification.

Fix the channel mapping by mapping bits 25 and 26 to RLC (Rear Left
Center) and RRC (Rear Right Center), respectively, instead.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/stream.c

@@ -273,8 +273,8 @@ static struct snd_pcm_chmap_elem *convert_chmap(int channels, unsigned int bits,
 		SNDRV_CHMAP_TSL,	/* top side left */
 		SNDRV_CHMAP_TSR,	/* top side right */
 		SNDRV_CHMAP_BC,		/* bottom center */
-		SNDRV_CHMAP_BLC,	/* bottom left center */
-		SNDRV_CHMAP_BRC,	/* bottom right center */
+		SNDRV_CHMAP_RLC,	/* back left of center */
+		SNDRV_CHMAP_RRC,	/* back right of center */
 		0 /* terminator */
 	};
 	struct snd_pcm_chmap_elem *chmap;