tg3: Use ext rx bds

The 5717 only uses extended buffer descriptors for the jumbo producer
ring.  Extended buffer descriptors are available on all devices that
support a separate jumbo producer ring so make the change universal.

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