tg3: Make 57791 and 57795 10/100 only

This patch adds the 57791 and 57795 to the list of devices that only
support 10 and 100 Mbps speeds.

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