lantiq: fix an ethernet stability issue triggered by receving packets during boot

Disabling ethernet during reboot (only to enable it again when the
ethernet driver attaches) can put the chip into a faulty state where it
corrupts the header of all incoming packets.

This happens if packets arrive during the time window where the core is
disabled, and it can be easily reproduced by rebooting while sending a
flood ping to the broadcast address.

Signed-off-by: Felix Fietkau <nbd@nbd.name>