drivers: eth: nxp,enet-qos: Fix MAC address check

Switch from `DT_INST_NODE_HAS_PROP(n, zephyr_random_mac_address)`
to `DT_INST_PROP(n, zephyr_random_mac_address)` since the property
is always present and we are interested in its value.

Signed-off-by: Maksim Salau <maksim.salau@gmail.com>