spi: spidev: Use SPI_MODE_USER_MASK instead of casting

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.com
Signed-off-by: Mark Brown <broonie@kernel.org>