spi: dw: Fix Designware SPI loopback

The SPI_LOOP is set in spi->mode but not propagated to the register.
A previous patch removed the bit during a cleanup.

Fixes: e1bc204 ("spi: dw: fix potential variable assignment error")
Signed-off-by: Thor Thayer <thor.thayer@linux.intel.com>
Link: https://lore.kernel.org/r/1572985330-5525-1-git-send-email-thor.thayer@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw.c

@@ -308,7 +308,8 @@ static int dw_spi_transfer_one(struct spi_controller *master,
 	cr0 = (transfer->bits_per_word - 1)
 		| (chip->type << SPI_FRF_OFFSET)
 		| ((((spi->mode & SPI_CPOL) ? 1 : 0) << SPI_SCOL_OFFSET) |
-			(((spi->mode & SPI_CPHA) ? 1 : 0) << SPI_SCPH_OFFSET))
+			(((spi->mode & SPI_CPHA) ? 1 : 0) << SPI_SCPH_OFFSET) |
+			(((spi->mode & SPI_LOOP) ? 1 : 0) << SPI_SRL_OFFSET))
 		| (chip->tmode << SPI_TMOD_OFFSET);
 
 	/*