spi: Replace if-else-if by bitops and multiplications

Instead of if-else-if, simply call roundup_pow_of_two(BITS_PER_BYTES()).
Note, there is no division assumed as compiler may optimize it away.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230710154932.68377-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>