net: enetc: move phylink_start/stop out of enetc_start/stop

We want to introduce a fast interface reconfiguration procedure, which
involves temporarily stopping the rings.

But we want enetc_start() and enetc_stop() to not restart PHY autoneg,
because that can take a few seconds until it completes again.

So we need part of enetc_start() and enetc_stop(), but not all of them.
Move phylink_start() right next to phylink_of_phy_connect(), and
phylink_stop() right next to phylink_disconnect_phy(), both still in
ndo_open() and ndo_stop().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>