Set/clear drive->waiting_for_dma flag in the core code
instead of in ->dma_setup and ->dma_end methods.

There should be no functional changes caused by this patch.
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>