ASoC: rsnd: make arrays path and cmd_case static const

Don't populate the arrays path and cmd_case on the stack but make
them static const.  Makes the object code smaller:

Before:
   text    data     bss     dec     hex filename
   2673     624       0    3297     ce1 sound/soc/sh/rcar/cmd.o

After:
   text    data     bss     dec     hex filename
   2398     768       0    3166     c5e sound/soc/sh/rcar/cmd.o

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