[PATCH] ahci: Ensure that we don't grab both functions

When we force the chip into dual fn mode so we get PATA and AHCI we must
be sure we don't then do anything dumb like try and grab both with the AHCI
driver.

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