sata_mv: warn on PIO with multiple DRQs

Chip errata sometimes prevents reliable use of PIO commands which involve
more than a single DRQ (data request).  In normal operation, libata should
not generate such PIO commands (uses DMA instead), but they could be sent
in via SG_IO from userspace.

A full workaround might be to break up such commands into sequences
of single DRQ ones, but that's just way too complex for something
that doesn't normally happen in real life.

So, allow the attempt (it often works, despite the errata),
but log the event for reference when somebody screams.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>