net: tsec: only call tsec_get_interface as fallback to DT-specified PHY mode

net: tsec: only call tsec_get_interface as fallback to DT-specified PHY mode

Currently the init_phy function may overwrite the priv->interface
property, since it calls tsec_get_interface which tries to determine it
dynamically based on default register values in ECNTRL.

Let's do that only if phy-connection-type happens to not be defined in
the device tree.

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