mmc: bcm2835: Drop DMA channel error pointer check

bcm2835_add_host() invokes IS_ERR_OR_NULL() on a DMA channel pointer,
however dma_request_slave_channel() (which was used to populate the
pointer) never returns an error pointer.  So a NULL pointer check is
sufficient.
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/bcm2835.c

@@ -1286,7 +1286,7 @@ static int bcm2835_add_host(struct bcm2835_host *host)
 	spin_lock_init(&host->lock);
 	mutex_init(&host->mutex);
 
-	if (IS_ERR_OR_NULL(host->dma_chan_rxtx)) {
+	if (!host->dma_chan_rxtx) {
 		dev_warn(dev, "unable to initialise DMA channel. Falling back to PIO\n");
 		host->use_dma = false;
 	} else {