tg3: Add support for 2 new selfboot formats

This patch adds new offsets to the bootcode version extraction code to
support NVRAM format versions 4 and 5.

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