spi: spi-imx: correctly configure burst length when using dma

If DMA is used, burst length should be set to the bus width of the DMA.
Otherwise, the SPI hardware will transmit/receive one word per DMA
request.
Since this issue affects both transmission and reception, it cannot be
detected with a loopback test.
Replace magic numbers 512 and 0xfff with MX51_ECSPI_CTRL_MAX_BURST.

Reported-by Stefan Bigler <linux@bigler.io>
Signed-off-by: Benjamin Bigler <benjamin@bigler.one>
Fixes: 15a6af94 ("spi: Increase imx51 ecspi burst length based on transfer length")
Link: https://lore.kernel.org/r/8a415902c751cdbb4b20ce76569216ed@mail.infomaniak.com
Link: https://lore.kernel.org/r/20231209222338.5564-1-benjamin@bigler.oneSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -659,12 +659,19 @@ static int mx51_ecspi_prepare_transfer(struct spi_imx_data *spi_imx,
 		ctrl |= (spi_imx->target_burst * 8 - 1)
 			<< MX51_ECSPI_CTRL_BL_OFFSET;
 	else {
-		if (spi_imx->count >= 512)
-			ctrl |= 0xFFF << MX51_ECSPI_CTRL_BL_OFFSET;
+		if (spi_imx->usedma) {
+			ctrl |= (spi_imx->bits_per_word *
+				spi_imx_bytes_per_word(spi_imx->bits_per_word) - 1)
+				<< MX51_ECSPI_CTRL_BL_OFFSET;
+		} else {
+			if (spi_imx->count >= MX51_ECSPI_CTRL_MAX_BURST)
+				ctrl |= (MX51_ECSPI_CTRL_MAX_BURST - 1)
+						<< MX51_ECSPI_CTRL_BL_OFFSET;
 			else
 				ctrl |= (spi_imx->count * spi_imx->bits_per_word - 1)
 						<< MX51_ECSPI_CTRL_BL_OFFSET;
 		}
+	}
 
 	/* set clock speed */
 	ctrl &= ~(0xf << MX51_ECSPI_CTRL_POSTDIV_OFFSET |