tg3: allow ethtool -p to work for NICs in down state

Make tg3 behave like other drivers and let "ethtool -p" identify the
NIC even when it's in the DOWN state. Before this patch it would get an
error as follows if the NIC was down:

# ip link set down dev em4
# ethtool -p em4
Cannot identify NIC: Resource temporarily unavailable

With this patch ethtool identify works regardless of whether the NIC is up
or down as it does for other drivers.

Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>