libata: Add a drivers/ide style DMA disable

This is useful when debugging, handling problem systems, or for
distributions just to get the system installed so it can be sorted
out later.

This is a bit smarter than the old IDE one and lets you do

libata.dma=0	Disable all PATA DMA like old IDE
libata.dma=1	Disk DMA only
libata.dma=2	ATAPI DMA only
libata.dma=4	CF DMA only

(or combinations thereof - 0,1,3 being the useful ones I suspect)

(I've split CF as it seems to be a seperate case of pain and suffering
different to the others and caused by assorted PIO wired adapters etc)

Signed-off-by: Alan Cox <alan@redhat.com>

[edited to work on SATA too, changing name from 'pata_dma' to 'dma']
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>