spi: bcm2835: added comment about different bus behaviour of DMA mode

The DMA mode behaves slightly different than polling or interrupt driven
mode, so just document the fact
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>

Changelog:
  V1 -> V2: applied feedback by Stefan Wahren
            new in V2
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -800,7 +800,10 @@ static int bcm2835_spi_transfer_one(struct spi_master *master,
 	if (tfr->len < spi_used_hz / HZ_PER_BYTE)
 		return bcm2835_spi_transfer_one_poll(master, spi, tfr, cs);
 
-	/* run in dma mode if conditions are right */
+	/* run in dma mode if conditions are right
+	 * Note that unlike poll or interrupt mode DMA mode does not have
+	 * this 1 idle clock cycle pattern but runs the spi clock without gaps
+	 */
 	if (master->can_dma && bcm2835_spi_can_dma(master, spi, tfr))
 		return bcm2835_spi_transfer_one_dma(master, spi, tfr, cs);