netfilter: xt_qtaguid: fix dev_stats for missing NETDEV_UNREGISTER

Turns out that some devices don't call the notifier chains
with NETDEV_UNREGISTER.
So now we only track up/down as the points for tracking
active/inactive transitions and saving the get_dev_stats().

Change-Id: I948755962b4c64150b4d04f294fb4889f151e42b
Signed-off-by: JP Abgrall <jpa@google.com>