mtd: spi-nor: nxp-spifi: Use SPI_MODE_X_MASK

Use SPI_MODE_X_MASK instead of open coded variant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>