spi: sprd: Make sure offset not equal to slave address size

The slave register offset shouldn't equal to the max slave address
which ADI can support to access.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20210824070212.2089255-2-zhang.lyra@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>