From: Jens Axboe <axboe@suse.de>

The blacklist stuff is broken. When set_using_dma() calls into
->ide_dma_check(), it returns __ide_dma_off() for a blacklisted drive.
This of course succeeds, returning success to the caller of ->ide_dma_check().
Not so good... It then uncondtionally calls ->ide_dma_on(), which turns on
DMA for the drive.

This moves the check to __ide_dma_on() so we also catch the buggy
->ide_dma_check() defined by various chipset drivers.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>