ASoC: fsl_spdif: make const arrays rate static

Don't populate the const arrays rate on the stack, instead make them
static. Makes the object code smaller by 220 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  24385	   9776	    128	  34289	   85f1	sound/soc/fsl/fsl_spdif.o

After:
   text	   data	    bss	    dec	    hex	filename
  24005	   9936	    128	  34069	   8515	sound/soc/fsl/fsl_spdif.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>