tg3: Add some VPD preprocessor constants

This patch cleans up the VPD code by creating preprocessor definitions
and using them in the place of hardcoded 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>