mmc: bcm2835: Properly handle dmaengine_prep_slave_sg

In case dmaengine_prep_slave_sg fails we need to call dma_unmap_sg.

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>