spi: Increase imx51 ecspi burst length based on transfer length

IMX51 supports 4096 bit burst lengths. Using the spi transfer length
instead of bits_per_word increases performance significantly.
Signed-off-by: Stefan Moring <stefan.moring@technolution.nl>
Link: https://lore.kernel.org/r/20230628125406.237949-1-stefan.moring@technolution.nlSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -659,9 +659,13 @@ static int mx51_ecspi_prepare_transfer(struct spi_imx_data *spi_imx,
 	if (spi_imx->slave_mode && is_imx53_ecspi(spi_imx))
 		ctrl |= (spi_imx->slave_burst * 8 - 1)
 			<< MX51_ECSPI_CTRL_BL_OFFSET;
-	else
-		ctrl |= (spi_imx->bits_per_word - 1)
-			<< MX51_ECSPI_CTRL_BL_OFFSET;
+	else {
+		if (spi_imx->count >= 512)
+			ctrl |= 0xFFF << MX51_ECSPI_CTRL_BL_OFFSET;
+		else
+			ctrl |= (spi_imx->count*8 - 1)
+				<< MX51_ECSPI_CTRL_BL_OFFSET;
+	}
 
 	/* set clock speed */
 	ctrl &= ~(0xf << MX51_ECSPI_CTRL_POSTDIV_OFFSET |
@@ -1258,6 +1262,7 @@ static int spi_imx_setupxfer(struct spi_device *spi,
 		spi_imx->spi_bus_clk = t->speed_hz;
 
 	spi_imx->bits_per_word = t->bits_per_word;
+	spi_imx->count = t->len;
 
 	/*
 	 * Initialize the functions for transfer. To transfer non byte-aligned