Add MWDMA support, and rework pio/mwdma/udma mode setup.

In the lone test case for PATA support, ata_piix, MWDMA mode setting
does not appear to work here.  UDMA and PIO continue to work, so nobody
will really notice.  But beware.  Probably a driver problem, not
a bug in the core.

Also, doesn't bother writing to dummy timing registers on ICH5/6 SATA
anymore.