tg3: Preserve register settings for DASH

Broadcom's DASH (Desktop and mobile Architecture for System Hardware)
implementation requires that the driver preserve particular register
settings.  If the driver does not preserve them, communication with
the DASH firmware will be lost.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>