spi: npcm-fiu: Reuse BITS_PER_BYTE definition

No need to redefine already existing definition.
So, replace custom by generic one.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200618170144.57433-1-andriy.shevchenko@linux.intel.com


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