tg3: Add reliable serdes detection for 5717 A0

The serdes status bit does not work as intended for the 5717 A0.
This patch implements an alternative detection scheme that will only be
valid for A0 revisions.

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