net: phy: mscc: macsec: reject PN update requests

Updating the PN is not supported.
Return -EINVAL if update_pn is true.

The following command succeeded, but it should fail because the driver
does not update the PN:
ip macsec set macsec0 tx sa 0 pn 232 on

Fixes: 28c5107a

 ("net: phy: mscc: macsec support")
Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>