octeontx2-pf: mcs: update PN only when update_pn is true

When updating SA, update the PN only when the update_pn flag is true.
Otherwise, the PN will be reset to its previous value using the
following command and this should not happen:
$ ip macsec set macsec0 tx sa 0 on

Fixes: c54ffc73

 ("octeontx2-pf: mcs: Introduce MACSEC hardware offloading")
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>