spi: davinci: fix SPI_NO_CS functionality

The driver should not touch CS lines if SPI_NO_CS flag is set.
This patch fixes it as this functionality was broken accidentally
by
commit a88e34ea213e1b ("spi: davinci: add support to configure gpio cs through dt").

Fixes: a88e34ea213e1b ("spi: davinci: add support to configure gpio cs through dt")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>