serverworks: cleanup ->set_dma_mode method

IDE core guarantees that ->set_dma_mode will be called only
for DMA modes set in SWDMA/MWDMA/UDMA masks.

There should be no functionality changes caused by this patch.

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