drivers: nxp_enet: Check link state in iface init

Still mark the iface as down after ethernet_init, but then actually
check the link state and initialize carrier appropriately

This fixes the case where, the phy driver doesn't give a callback after
iface init due to the link already being up, there was no change from
the phy driver perspective, so callback  wouldn't happen, and therefore
the interface could remain marked as down after boot even if carrier is up.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>