serial: pch_uart: Update error message 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 an error message.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Jiri Kosina <trivial@kernel.org>
Cc: linux-serial@vger.kernel.org
--
v2:
  - New
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>