mmc: bcm2835: Release DMA channel on driver unload

We need to release the slave DMA channel during driver unload.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>