tg3: Prepare FW version code for VPD versioning

The code that extracts the firmware version from the device's NVRAM
assumes the firmware version member is a clean slate.  The following
patch will add code to extract the firmware version from the VPD area
of NVRAM, so this assumption will no longer be true.  This patch adjusts
the versioning code to respect the VPD version if it exists.

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