net: enetc: require a PHY device when probing

Given that even a fixed-link has an associated phy_device, there is no
reason to operate in a mode when dm_eth_phy_connect fails.

Remove the driver checks for a NULL priv->phy and just return -ENODEV
when that happens.

Copyright updated according to corporate requirements.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>