macsec: fix error codes when a SA is created

preserve the return value of AEAD functions that are called when a SA is
created, to avoid inappropriate display of "RTNETLINK answers: Cannot
allocate memory" message.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>