mfd: wm8994: Use devm_regulator_bulk_get API

devm_regulator_bulk_get is device managed and saves some cleanup
and exit code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>