[MAC80211]: use internal network device stats

Stats are now available for device usage inside network_device

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>