mmc: bcm2835: Terminate timeout work synchronously

It's better to make sure that the timeout work is really terminated
before calling mmc_request_done.

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>