ALSA: vx: vx_pcm: remove redundant assignment

Fix cppcheck warning:

sound/drivers/vx/vx_pcm.c:63:7: style: Variable 'buf' is assigned a
value that is never used. [unreadVariable]
  buf = (unsigned char *)runtime->dma_area;
      ^
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902212133.30964-19-pierre-louis.bossart@linux.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/vx/vx_pcm.c

@@ -60,7 +60,6 @@ static void vx_pcm_read_per_bytes(struct vx_core *chip, struct snd_pcm_runtime *
 	*buf++ = vx_inb(chip, RXL);
 	if (++offset >= pipe->buffer_bytes) {
 		offset = 0;
-		buf = (unsigned char *)runtime->dma_area;
 	}
 	pipe->hw_ptr = offset;
 }