mmc: bcm2835: Avoid possible races on data requests

There are two accesses on the data requests which are not protected by
the mutex. So fix this accordingly.

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>