libata: Correct IORDY handling

Debugging a report of a problem with an ancient solid state disk showed
up some problems in the IORDY handling

1.	We check the wrong bit to see if the device has IORDY
2.	Even then some ancient creaking piles of crap don't support
	SETXFER at all.

The cases it fixes are obscure and the risk of side effects is slight
but possible. This also moves us slightly closer to supporting original
MFM/RLL disks with libata.

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