ns87415: use custom ->dma_{start,end} to handle ns87415_prepare_drive()

Use custom ->dma_{start,end} methods to handle ns87415_prepare_drive()
there instead of in ->dma_setup method.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>