mmc: bcm2835: Refactor dma_map_sg handling

There are two variables len within bcm2835_prepare_dma. So rename the
result of dma_map_sg to sg_len. While we are at this add a bail out to
simplify the following change.

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>