spi: armada-3700: Set mode bits correctly

We set SPI_RX_DUAL twice instead of setting SPI_TX_DUAL.

Fixes: 5762ab71 ("spi: Add support for Armada 3700 SPI Controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>