ASoC: Constify reg_default tables

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Peter Rosin <peda@axentia.se>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>