sata_nv: handle SError status indication

ADMA-capable controllers provide a bit in the status register that appears
to indicate that the controller detected an SError condition. Update sata_nv
to detect this and trigger error handling in order to handle the fault.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>