tg3: Lay proucer ring handling groundwork

The patch increases the number of producer rings available and
implements the constructor and destructor code that deals with them.

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