spi: pic32: Set proper bits_per_word_mask

This driver only supports 8/16/32 bits_per_word, so set
master->bits_per_word_mask accordingly. With this change, we can remove
the spi->bits_per_word checking in pic32_spi_setup as it's done by spi
core.

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