ASoC: sgtl5000: add one missed cache reg

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Zeng Zhaoming <zhaoming.zeng@freescale.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>