ASoC: wm8997: Fix compile error

The macro's name is SOC_ENUM, not SOC_VALUE.

Fixes: e13dd8ce ("ASoC: wm8997: Replace usage deprecated MUX/ENUM macros")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>