Maintain a local (mv_port_priv) cache of frequently accessed registers,
to avoid having to re-read them (very slow) on every transistion
between EDMA and non-EDMA modes.  This speeds up things like
flushing the drive write cache, and anything using basic DMA transfers.
Signed-off-by: NMark Lord <mlord@pobox.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>