libata: Be a bit more slack about early devices

We have a certain number of 'ATA' emulations often on CF or other flash
devices that are at best "loosely based" on the CF 1.1 standard. These
devices report themselves as disk but don't support the ATA minimal
command set only the CF 1.1 set.

Relax the PIO checking for devices reporting ATA rev 0, or no iordy
support, or CFA. Rework the code a bit as it was already messy and this
made it quite ugly.

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