tg3: Use devfn to determine function number

The driver sometimes needs to know which function number the current
device is.  This patch changes the code to use devfn over internal
register values.

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