IDE changes that affect upper layer drivers

[Alan Cox]

I'm about to enable the vmda logic for non disk drivers. That means IDE
tape, scsi, cd and friends need updating to follow the new rules

Before it was simply:
	->dma = 1   - do DMA

Some devices and a lot of upcoming ones support DMA to the host while
doing PIO to the device "Virtual DMA". That means the drivers now
need to do

	DMA	VDMA
	 0	  X		PIO
	 1	  0		Issue PIO commands, set up for DMA xfers
	 1        1		Issue DMA commands, set up for DMA xfers

Alan