ALSA: sound/core/pcm_compat.c: adjust array index

Convert array index from the loop bound to the loop index.

A simplified version of the semantic patch that fixes this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e1,e2,ar;
@@

for(e1 = 0; e1 < e2; e1++) { <...
  ar[
- e2
+ e1
  ]
  ...> }
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Cc: <stable@kernel.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/pcm_compat.c

@@ -342,7 +342,7 @@ static int snd_pcm_ioctl_xfern_compat(struct snd_pcm_substream *substream,
 			kfree(bufs);
 			return -EFAULT;
 		}
-		bufs[ch] = compat_ptr(ptr);
+		bufs[i] = compat_ptr(ptr);
 		bufptr++;
 	}
 	if (dir == SNDRV_PCM_STREAM_PLAYBACK)