spi: fsl-espi: expose maximum transfer size limit

The fsl-espi hardware can trasfer at most 64K data so report teh
limitation.

Based on patch by Heiner Kallweit <hkallweit1@gmail.com>

CC: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>