spi: nuc900: Fix setting multiple bits settings in register

The correct way to set multiple bits settings is always clear these
bit fields before set new settings.

Current code does not cause problem because the reset value of these
bit fields are 0, and these settings only set once during probe.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>