ide: remove redundant ->ide_dma_on call from set_using_dma()

ide_set_dma() calls ->ide_dma_on method itself and returns zero
only if ->ide_dma_on call succeeded.

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>