ide: remove redundant DMA blacklist check from __ide_dma_on()

->ide_dma_on method is called only after successful ide_dma_check() call
(ide_dma_check()->ide_tune_dma() checks DMA blacklist) or if drive->using_dma
has been previously enabled for a given device (->ide_dma_on is the only place
which sets drive->using_dma to '1').

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>