net: macsec: add support for getting offloaded stats

When HW offloading is enabled, offloaded stats should be used, because
s/w stats are wrong and out of sync with the HW in this case.

Signed-off-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>