ASoC: wm2200: Correct types of mixer texts and values

The core expects "const char * const" and "unsigned int" for enum
controls, the mixer control definitions in wm2200 use "const char *"
and "int".  This patch corrects the type of these arrays.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>