spi: dw: Fix default polarity of native chipselect

The DW controller also supports platforms specifying
native chipselects. When I enforce the use of high CS
for drivers opting in for using GPIO descriptors, I
inadvertedly switched the driver to also use active
high chip select for native chip selects.

As it turns out, the DW hardware driving chip selects
also thinks it is weird with active low chip selects
so all we need to do is remove an inversion in the
driver.

Cc: Jan Kotas <jank@cadence.com>
Reported-by: Jan Kotas <jank@cadence.com>
Tested-by: Jan Kotas <jank@cadence.com>
Fixes: 9400c41e

 ("spi: dw: Convert to use CS GPIO descriptors")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>