spi: uniphier: fix wrong register overwrite

When it changes the spi mode, the register is overwritten incorrectly.
This commit fixes this register overwrite.

Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Link: https://lore.kernel.org/r/1567488661-11428-2-git-send-email-hayashibara.keiji@socionext.com


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