spi: spi-synquacer: fix set_cs handling

When the slave chip select is deasserted, DMSTOP bit
must be set.

Fixes: b0823ee3

 ("spi: Add spi driver for Socionext SynQuacer platform")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210201073109.9036-1-jassisinghbrar@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>