spi: fsl-espi: reject MSB-first transfers with word sizes other than 8 or 16

According to the ESPI spec MSB-first transfers are supported for
word size 8 and 16 only.

Check for this and reject MSB-first transfers with other word sizes.

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