ASoC: soc-cache: let reg be AND'ed by 0xff instead of data buffer for 8_8 mode

The registers for AD193X are defined as 0x800-0x810 for spi which uses
16_8 mode, for i2c to support AD1937, we will use 8_8 mode, only the low
byte of 0x800-0x810 is valid.  The patch will not destory other codecs,
but make soc cache interface more useful.

Signed-off-by: Barry Song <barry.song@analog.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>