sata_mv: fis irq register fixes

Fix handling of the FIS_IRQ_CAUSE register in sata_mv.

This register exists *only* on GenIIe devices, so don't bother
writing to it on older chips.  Also, it has to be read/cleared
in mv_err_intr() before clearing the main ERR_IRQ_CAUSE register.

This keeps sata_mv from getting stuck forever on certain error types.

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