spi: rspi: Use dev_warn_once() instead of open-coding

Use the helper introduced by commit e135303b ("device: Add
dev_<level>_once variants") instead of open-coding the same operation.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-4-geert+renesas@glider.beSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-rspi.c

@@ -620,9 +620,8 @@ static int rspi_dma_transfer(struct rspi_data *rspi, struct sg_table *tx,
 		dmaengine_terminate_all(rspi->ctlr->dma_rx);
 no_dma_rx:
 	if (ret == -EAGAIN) {
-		pr_warn_once("%s %s: DMA not available, falling back to PIO\n",
-			     dev_driver_string(&rspi->ctlr->dev),
-			     dev_name(&rspi->ctlr->dev));
+		dev_warn_once(&rspi->ctlr->dev,
+			      "DMA not available, falling back to PIO\n");
 	}
 	return ret;
 }