spi: Don't mark message DMA mapped when no transfer in it is

There is no need to set the DMA mapped flag of the message if it has
no mapped transfers. Moreover, it may give the code a chance to take
the wrong paths, i.e. to exercise DMA related APIs on unmapped data.
Make __spi_map_msg() to bail earlier on the above mentioned cases.

Fixes: 99adef31 ("spi: Provide core support for DMA mapping transfers")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://msgid.link/r/20240522171018.3362521-2-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1243,6 +1243,7 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
 	else
 		rx_dev = ctlr->dev.parent;
 
+	ret = -ENOMSG;
 	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
 		/* The sync is done before each transfer. */
 		unsigned long attrs = DMA_ATTR_SKIP_CPU_SYNC;
@@ -1272,6 +1273,9 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
 			}
 		}
 	}
+	/* No transfer has been mapped, bail out with success */
+	if (ret)
+		return 0;
 
 	ctlr->cur_rx_dma_dev = rx_dev;
 	ctlr->cur_tx_dma_dev = tx_dev;