mlxsw: spectrum: use netif_is_macsec() instead of open code

Open code which is dev->priv_flags & IFF_MACSEC has already defined as
netif_is_macsec(). So use netif_is_macsec() instead of open code.
This patch doesn't change logic.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>