aquantia: Fix "ethtool -S" crash when adapter down.

This patch fixes the crash that happens when driver tries to collect statistics
from already released "aq_vec" object.
If adapter is in "down" state we still allow user to see statistics from HW.

V2: fixed braces around "aq_vec_free".

Fixes: 97bde5c4

 ("net: ethernet: aquantia: Support for NIC-specific code")
Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Tested-by: David Arcari <darcari@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>