Fix a number of issues with the DMA support code:

- driver claims support for all SW/MW DMA modes while supporting only MWDMA2;

- ide_dma_check() method tries to enable DMA on the "known good" drives which
  don't support MWDMA2;

- ide_dma_on() method upon failure to set drive to MWDMA2 re-tunes already
  tuned PIO mode and calls ide_dma_off() method instead of returning error;

- ide_dma_off() method sets drive->current_speed while it doesn't actually
  change (only the PIO timings are re-loaded into the chip's registers);

- init_hwif() method forcibly sets/resets both "drive DMA capable" bits while
  this is properly handled by ide_dma_{on,off}() methods being called later...
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>