spi: fsl-spi: Convert to let spi core validate xfer->bits_per_word

Set bits_per_word_mask so spi core will reject transfers that attempt to use
an unsupported bits_per_word value.

fsl_spi_grlib_probe() may update mpc8xxx_spi->max_bits_per_word setting.
So set master->bits_per_word_mask after fsl_spi_grlib_probe().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>