ionic: fix stats memory dereference

When the netdev is down, the queues and their debug stats
do not exist, so don't try using a pointer to them when
when printing the ethtool stats.

Fixes: e470355b ("ionic: Add driver stats")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>