spi: spidev: Use SPI_MODE_USER_MASK instead of casting

Currently the 16-bit mode is what being used in user space.
However assuming that is not fully correct. Instead we should
use the respective mask, i.e. SPI_MODE_USER_MASK, which
precisely defines what bits are available for user space apps.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211122175245.84691-1-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spidev.c

@@ -415,7 +415,7 @@ spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 				tmp |= SPI_CS_HIGH;
 
 			tmp |= spi->mode & ~SPI_MODE_MASK;
-			spi->mode = (u16)tmp;
+			spi->mode = tmp & SPI_MODE_USER_MASK;
 			retval = spi_setup(spi);
 			if (retval < 0)
 				spi->mode = save;