net/mlx5e: macsec: use update_pn flag instead of PN comparation

When updating the SA, use the new update_pn flags instead of comparing the
new PN with the initial one.

Comparing the initial PN value with the new value will allow the user
to update the SA using the initial PN value as a parameter like this:
$ ip macsec add macsec0 tx sa 0 pn 1 on key 00 \
ead3664f508eb06c40ac7104cdae4ce5
$ ip macsec set macsec0 tx sa 0 pn 1 off

Fixes: 8ff0ac5b ("net/mlx5: Add MACsec offload Tx command support")
Fixes: aae3454e ("net/mlx5e: Add MACsec offload Rx command 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>