spi: spi-mem: Adjust op len based on message/transfer size limitations

We need that to adjust the len of the 2nd transfer (called data in
spi-mem) if it's too long to fit in a SPI message or SPI transfer.

Fixes: c36ff266 ("spi: Extend the core to ease integration of SPI memory controllers")
Cc: <stable@vger.kernel.org>
Signed-off-by: Chuanhua Han <chuanhua.han@nxp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>