spi: npcm-fiu: Fix UMA reads when dummy.nbytes == 0

We don't want to use the value of ilog2(0) as dummy.buswidth is 0 when
dummy.nbytes is 0. Since we have no dummy bytes, we don't need to
configure the dummy byte bits per clock register value anyway.

Signed-off-by: "William A. Kennington III" <william@wkennington.com>
Link: https://lore.kernel.org/r/20230922182812.2728066-1-william@wkennington.com
Signed-off-by: Mark Brown <broonie@kernel.org>