mfd: wm8994-regmap: Constify reg_default tables

These reg_default tables are not modified after initialized, so make them
const.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>