spi: spidev: mask SPI_CS_HIGH in SPI_IOC_RD_MODE

spi: spidev: mask SPI_CS_HIGH in SPI_IOC_RD_MODE

Commit f3186dd87669 ("spi: Optionally use GPIO descriptors for CS GPIOs")
has changed the user-space interface so that bogus SPI_CS_HIGH started
to appear in the mask returned by SPI_IOC_RD_MODE even for active-low CS
pins. Commit 138c9c32f090
("spi: spidev: Fix CS polarity if GPIO descriptors are used") fixed only
SPI_IOC_WR_MODE part of the problem. Let's fix SPI_IOC_RD_MODE
symmetrically.

Test case:

	#include <sys/ioctl.h>
	#include <fcntl.h>
	#include <linux/spi/spidev.h>

	int main(int argc, char **argv)
	{
		char modew = SPI_CPHA;
		char moder;
		int f = open("/dev/spidev0.0", O_RDWR);

		if (f < 0)
			return 1;

		ioctl(f, SPI_IOC_WR_MODE, &modew);
		ioctl(f, SPI_IOC_RD_MODE, &moder);

		return moder == modew ? 0 : 2;
	}

Fixes: f3186dd87669 ("spi: Optionally use GPIO descriptors for CS GPIOs")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://lore.kernel.org/r/20221130162927.539512-1-alexander.sverdlin@siemens.com
Signed-off-by: Mark Brown <broonie@kernel.org>