spi: fsl-espi: remove usage of mpc8xxx_spi->flags

Change the check to access property "mode" directly.
This allows us to get rid of mpc8xxx_spi->flags in a subsequent
patch in this patch series as it's used nowhere else.

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