ethoc: 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.

Fixed ethoc_set_mac_address() to check if the given mac
address is valid and set also dev_addr of the net_device.
Check also the return value of ethoc_set_mac_address() in
ethoc_probe().

Reset the state to NET_ADDR_PERM as soon as the MAC get
changed via .ndo_set_mac_address.

v2: set net_device->dev_addr in ethoc_set_mac_address(),
    check if given address is valid

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>