pata_via: Cache and rewrite the device bit

Some VIA chipsets will reset the DEV bit after IEN changes on ctl. Our
optimised write path avoids doing this but we need to remove the
optimisation on these devices.

[Identified and some original patches proposed by Josehn Chan @ VIA but
discussion then all ground to a halt so given a test case I dug it back out]

Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Tested-by: Christoph Bisping (bug #13086)
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>