nvmem: sunxi-sid: fix H3 SID controller support

It seems that doing some operation will make the value pre-read on H3
SID controller wrong again, so all operation should be performed by
register.

Change the SID reading to use register only.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>