spi: spi-imx: fix off-by-one in mx51 CPU mode burst length

c712c05e ("spi: imx: fix the burst length at DMA mode and CPU mode")
corrects three cases of setting the ECSPI burst length but erroneously
leaves the in-range CPU case one bit to big (in that field a value of
0 means 1 bit).  The effect was that transmissions that should have been
8-bit bytes appeared as 9-bit causing failed communication with SPI
devices.

Link: https://lore.kernel.org/all/20240201105451.507005-1-carlos.song@nxp.com/
Link: https://lore.kernel.org/all/20240204091912.36488-1-carlos.song@nxp.com/
Fixes: c712c05e ("spi: imx: fix the burst length at DMA mode and CPU mode")
Signed-off-by: Adam Butcher <adam@jessamine.co.uk>
Link: https://msgid.link/r/20240318175119.3334-1-adam@jessamine.co.ukSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -668,8 +668,8 @@ static int mx51_ecspi_prepare_transfer(struct spi_imx_data *spi_imx,
 				ctrl |= (MX51_ECSPI_CTRL_MAX_BURST * BITS_PER_BYTE - 1)
 						<< MX51_ECSPI_CTRL_BL_OFFSET;
 			else
-				ctrl |= spi_imx->count / DIV_ROUND_UP(spi_imx->bits_per_word,
-						BITS_PER_BYTE) * spi_imx->bits_per_word
+				ctrl |= (spi_imx->count / DIV_ROUND_UP(spi_imx->bits_per_word,
+						BITS_PER_BYTE) * spi_imx->bits_per_word - 1)
 						<< MX51_ECSPI_CTRL_BL_OFFSET;
 		}
 	}