spi: sun6i: Set bits_per_word_mask to only support 8 bits word length

This controller only supports 8 bits word length.
Set bits_per_word_mask so spi core will reject transfers that attempt to use
an unsupported bits_per_word value.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-sun6i.c

@@ -391,6 +391,7 @@ static int sun6i_spi_probe(struct platform_device *pdev)
 	master->transfer_one = sun6i_spi_transfer_one;
 	master->num_chipselect = 4;
 	master->mode_bits = SPI_CPOL | SPI_CPHA | SPI_CS_HIGH | SPI_LSB_FIRST;
+	master->bits_per_word_mask = SPI_BPW_MASK(8);
 	master->dev.of_node = pdev->dev.of_node;
 	master->auto_runtime_pm = true;