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.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>