spi: pxa2xx: Use predefined mask when programming FIFO thresholds

The predefined mask for threshold modification can be used
in case of Intel Merrifield SPI. Replace open-coded value
with predefined mask when programming FIFO thresholds.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210517140351.901-10-andriy.shevchenko@linux.intel.com


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