spi: mpc52xx: Convert to use bits_per_word_mask

This controller only supports 8-bit word length.
Set bits_per_word_mask so spi core will reject transfers that attempt to use
an unsupported bits_per_word value.
Also remove the duplicate code to test spi->mode, it is done by spi core.

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