mmc: bcm2835: Fix DMA channel leak on probe error

The BCM2835 MMC host driver requests a DMA channel on probe but neglects
to release the channel in the probe error path.  The channel may
therefore be leaked, in particular if devm_clk_get() causes probe
deferral.  Fix it.

Fixes: 660fc733

 ("mmc: bcm2835: Add new driver for the sdhost controller.")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v4.12+
Cc: Frank Pavlic <f.pavlic@kunbus.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>