spi: fsl-espi: improve check for SPI_QE_CPU_MODE

SPI_QE_CPU_MODE doesn't exist for ESPI and is set by of_mpc8xxx_spi_probe
based on DT property "mode". This property is not defined for ESPI,
see Documentation/devicetree/bindings/spi/fsl-spi.txt.
So print an error message and bail out if SPI_QE_CPU_MODE is set.

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