ide-pmac: remove control register messing from pmac_ide_dma_check()

pmac_ide_do_setfeature() contains matching nIEN setting/clearing so this
Device Control register messing in pmac_ide_dma_check() is totally unnecessary.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>