spi: bcm2835: Use SPI_BPW_MASK macro for bits_per_word_mask setting

We have a SPI_BPW_MASK macro defined in spi.h, use it instead of open-coded.

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