sata_mv: workaround errata SATA#13

Add remainder of workaround for errata SATA#13.

This prevents writes of certain adjacent 32-bit registers
from being combined into single 64-bit writes, which might
fail for the affected registers.

Most of sata_mv is already safe from this issue,
but adding this code to mv_write_cached_reg() will
catch the remaining cases and hopefully prevent future ones.

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