spi: Use BITS_TO_BYTES()

BITS_TO_BYTES() is the existing macro which takes care about full
bytes that may fully hold the given amount of bits. Use it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230714091748.89681-4-andriy.shevchenko@linux.intel.com


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