ASoC: sgtl5000: Fix wrong register name in restore

Correct SGTL5000_CHIP_CLK_CTRL to SGTL5000_CHIP_REF_CTRL in
sgtl5000_restore_regs(), and add comment to explain the
restore order.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Zeng Zhaoming <zengzm.kernel@gmail.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>