spi: cadence: Fix default polarity of native chipselect

The Cadence 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.

Fix this by inverting the logic in the callback for the
native chip select. Rename the parameter from "is_high"
(which is interpreted as being high when 0, which is
confusing, I will not make any drug-related jokes here)
to "enabled" which is more intuitive, especially now that
it is true when CS is supposed to be enabled.

Cc: Wei Yongjun <weiyongjun1@huawei.com>
Fixes: cfeefa79 ("spi: cadence: Convert to use CS GPIO descriptors")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>