sata_mv: leave SError bits untouched in mv_err_intr

Here it is again, minus the checkpatch.pl complaint:

Rework mv_err_intr() to leave the SError bits as-is,
so that libata-eh has a chance to see/use them.

We originally thought that clearing them here was necessary
before writing back to edma_err_cause (per the Marvell datasheets),
but we will end up reseting the chip regardless in those cases.

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