ASoC: Set sgtl5000->ldo in ldo_regulator_register

Otherwise calling ldo_regulator_remove() does not unregister regulator
and free memories.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>