spi: Switch to signed types for *_native_cs SPI controller fields

spi: Switch to signed types for *_native_cs SPI controller fields

While fixing undefined behaviour the commit f60d7270c8a3 ("spi: Avoid
undefined behaviour when counting unused native CSs") missed the case
when all CSs are GPIOs and thus unused_native_cs will be evaluated to
-1 in unsigned representation. This will falsely trigger a condition
in the spi_get_gpio_descs().

Switch to signed types for *_native_cs SPI controller fields to fix above.

Fixes: f60d7270c8a3 ("spi: Avoid undefined behaviour when counting unused native CSs")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210510131242.49455-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>