spi: fsl-espi: replace of_get_property with of_property_read_u32

of_property_read_u32 is better here than generic of_get_property:
- implicit endianness conversion if needed
- implicit checking of size of property

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