ALSA: vx_core: remove warning for empty loop body

Fix W=1 warning.

sound/drivers/vx/vx_core.c: In function ‘snd_vx_threaded_irq_handler’:
sound/drivers/vx/vx_core.c:515:3: warning: suggest braces around empty
body in an ‘if’ statement [-Wempty-body]
  515 |   ; /* so far, nothing to do yet */
      |   ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-23-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>