tg3: Create TG3_FLG3_5717_PLUS flag

This patch creates a TG3_FLG3_5717_PLUS flag to collectively describe
the set of changes in the ASIC that will apply to all future chip
revisions.

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