tg3: Add legacy bootcode version decoding

This patch adds code to obtain the bootcode version for versions
of bootcode that do not use the new method.

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