net: macsec: fix rtnl locking issue

netdev_update_features() must be called with the rtnl lock taken. Not
doing so triggers a warning, as ASSERT_RTNL() is used in
__netdev_update_features(), the first function called by
netdev_update_features(). Fix this.

Fixes: c850240b

 ("net: macsec: report real_dev features when HW offloading is enabled")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>