tg3: Prepare for larger rx ring sizes

This patch adds two new variables to track the size of the standard and
jumbo rx producer ring sizes.  The code is then pivoted to these
variables from preprocessor constants.

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