Remove unused vol from struct access_mask

Reduces object size ~3kb.

$ size sound/soc/codecs/wm8994.o*
   text	   data	    bss	    dec	    hex	filename
  40727	   4384	   4480	  49591	   c1b7	sound/soc/codecs/wm8994.o.new
  43879	   4384	   4480	  52743	   ce07	sound/soc/codecs/wm8994.o.old
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>