[libata] sata_mv: don't touch reserved bits in EDMA config register

The code in mv_edma_cfg() reflected its 60xx origins, by doing things
[slightly] incorrectly on the older 50xx and newer 6042/7042 chips.

Clean up the EDMA configuration setup such that, each chip family
carefully initializes its own EDMA setup.

Signed-off-by: Jeff Garzik <jeff@garzik.org>