netdev: octeon_mgmt: Use proper MAC addresses.

The original implementation incorrectly uses netdev->dev_addrs.

Use netdev->uc instead.  Also use netdev_for_each_uc_addr to iterate
over the addresses.  Fix comment.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>