tg3: Cleanup extended rx ring size code

Hardcoded values are used in multiple places to describe the maximum rx
ring sizes.  This patch replaces those values with preprocessor
constants.  This patch also introduces a new TG3_FLG3_LRG_PROD_RING_CAP
to determine if the device is capable of supporting larger ring sizes.

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