bcm2835-mmc: Fix DMA channel leak

The BCM2835 MMC host driver requests a DMA channel on probe but neglects
to release the channel in the probe error path and on driver unbind.

I'm seeing this happen on every boot of the Compute Module 3: On first
driver probe, DMA channel 2 is allocated and then leaked with a "could
not get clk, deferring probe" message. On second driver probe, channel 4
is allocated.

Fix it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>