tg3: 5717_PLUS => 57765_PLUS

The 57765 arrived before the 5717 and has a subset of the features
supported by the 5717.  This patch renames the 5717_PLUS flag so that it
can be reintroduced to designate only 5717 and later devices.

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