ASoC: samsung: smdk_wm8994: Fix build error

Fixes the following build error and warning when OF is not defined:
sound/soc/samsung/smdk_wm8994.c:191:23: error: ‘samsung_wm8994_of_match’ undeclared (first use in this function)
sound/soc/samsung/smdk_wm8994.c:47:32: warning: ‘smdk_board_data’ defined but not used [-Wunused-variable]

of_match_ptr() is used so that samsung_wm8994_of_match gets dropped (as unused)
by the compiler when OF is not defined.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>