dmaengine: imx-sdma: Fix the event id check to include RX event for UART6

On i.MX6UL/ULL and i.MX6SX the DMA event id for the RX channel of
UART6 is '0'. To fix the broken DMA support for UART6, we change
the check for event_id0 to include '0' as a valid id.

Fixes: 1ec1e82f ("dmaengine: Add Freescale i.MX SDMA support")
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200225082139.7646-1-frieder.schrempf@kontron.deSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/imx-sdma.c

@@ -1331,7 +1331,7 @@ static void sdma_free_chan_resources(struct dma_chan *chan)
 
 	sdma_channel_synchronize(chan);
 
-	if (sdmac->event_id0)
+	if (sdmac->event_id0 >= 0)
 		sdma_event_disable(sdmac, sdmac->event_id0);
 	if (sdmac->event_id1)
 		sdma_event_disable(sdmac, sdmac->event_id1);
@@ -1632,7 +1632,7 @@ static int sdma_config(struct dma_chan *chan,
 	memcpy(&sdmac->slave_config, dmaengine_cfg, sizeof(*dmaengine_cfg));
 
 	/* Set ENBLn earlier to make sure dma request triggered after that */
-	if (sdmac->event_id0) {
+	if (sdmac->event_id0 >= 0) {
 		if (sdmac->event_id0 >= sdmac->sdma->drvdata->num_events)
 			return -EINVAL;
 		sdma_event_enable(sdmac, sdmac->event_id0);