macsec: fix error message in macsec_add_rxsa and _txsa

The expected length is MACSEC_SALT_LEN, not MACSEC_SA_ATTR_SALT.

Fixes: 48ef50fa ("macsec: Netlink support of XPN cipher suites (IEEE 802.1AEbw)")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>