macsec: set actual real device for xmit when !protect_frames

Avoid recursions of dev_queue_xmit() to the wrong net device when
frames are unprotected, since at that time skb->dev still points to
our own macsec dev and unlike macsec_encrypt_finish() dev pointer
doesn't get updated to real underlying device.

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>