ASoC: SSM2602: Fix reg_cache_size

reg_cache_size is supposed to be the number of elements in the register cache,
not the size in bytes.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>