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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spidev.c

···
 				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;