lantiq_etop: set addr_assign_type if random_ether_addr() used

Set addr_assign_type correctly to NET_ADDR_RANDOM in case
a random MAC address was generated and assigned to the netdevice.

v2: added comment, renamed bool variable to random_mac

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: David S. Miller <davem@davemloft.net>