bcm2835-mmc: Prevent DMA race condition

The end of a read operation is triggered by the completion of the DMA
transfer, but writes are complete when the data IRQ is raised. The
bcm2835-mmc driver contains a race between the handling of the DMA
completion interrupt and the submission of the next request. Fix the
race by deferring the completion of the request until the DMA
transfer finishes.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>