tg3: Consolidate ASIC rev detection code

Detecting the ASIC revision of a device is getting to be an increasingly
complex process.  This patch consolidates all the ASIC rev detection
code to a single routine for better maintainability.

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