tg3: Refine VAux decision process

In the near future, the VAux switching decision process is going to get
more complicated.  This patch refines and consolidates the existing
algorithm in anticipation of the new scheme.

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