spi: dw: Use SPI_TMOD_TR rather than magic const 0 to set tmode

The TMODE available value is well defined and documented in the header
file. Use it and remove the comment.
Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -425,7 +425,7 @@ static int dw_spi_setup(struct spi_device *spi)
 		chip->type = chip_info->type;
 	}
 
-	chip->tmode = 0; /* Tx & Rx */
+	chip->tmode = SPI_TMOD_TR;
 
 	if (gpio_is_valid(spi->cs_gpio)) {
 		ret = gpio_direction_output(spi->cs_gpio,