spi: sprd: fill offset only to RD_CMD register for reading from slave device

RD_CMD can accept slave address offset only, higher bits are reserved.
Writing the whole slave address including slave base seems unnecessary.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lore.kernel.org/r/20210824070212.2089255-3-zhang.lyra@gmail.com


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