reenable DMA

[kenneth johansson <ken@kenjo.org>]

I have a small problem where my DVD player got confused by a disk
and the ata driver ended up putting the driver in PIO mode. 

Now I need to get it back into DMA without rebooting the computer. 

 hdparm -d1 /dev/sr0

/dev/sr0:
 setting using_dma to 1 (on)
 HDIO_SET_DMA failed: Inappropriate ioctl for device
 HDIO_GET_DMA failed: Inappropriate ioctl for device
-------
 ata9.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
 ata9.00: cmd a0/00:00:00:00:fc/00:00:00:00:00/a0 tag 0 pio 131072 in
          cdb 28 00 00 13 c6 4a 00 00  40 00 00 00 00 00 00 00
          res 40/00:02:00:08:00/00:00:00:00:00/a0 Emask 0x4 (timeout)
 ata9.00: status: { DRDY }
 ata9: soft resetting link
 ata9.00: configured for PIO0
 ata9: EH complete