spi: rb4xx: Use SPI_BPW_MASK to set bits_per_word_mask

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

drivers/spi/spi-rb4xx.c

@@ -159,7 +159,7 @@ static int rb4xx_spi_probe(struct platform_device *pdev)
 	master->bus_num = 0;
 	master->num_chipselect = 3;
 	master->mode_bits = SPI_TX_DUAL;
-	master->bits_per_word_mask = BIT(7);
+	master->bits_per_word_mask = SPI_BPW_MASK(8);
 	master->flags = SPI_MASTER_MUST_TX;
 	master->transfer_one = rb4xx_transfer_one;
 	master->set_cs = rb4xx_set_cs;