serverworks: fix DMA

The driver used to depend on BIOS settings for deciding whether it is OK
to use DMA.  However it seems that BIOS doesn't always handle all cases
correctly so just let IDE core to decide about this.  It should be a safe
thing to do now, after the driver went through heavy bugfixing.

Thanks for bugreport and testing the patch goes out to Sven Niedner.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>