spi: topcliff-pch: Update error messages for dmaengine_prep_slave_sg() API

Commit 16052827

 ("dmaengine/dma_slave:
introduce inline wrappers") changed the code to use the new API, but forgot
to update the error messages.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
--
v2:
  - New
Signed-off-by: Mark Brown <broonie@linaro.org>