stmmac: convert unicast addr list to list_head

This patch converts unicast address list to standard list_head using
previously introduced struct netdev_hw_addr.

Note: this patch also removes a debug printk used for displaying the
mac addresses. Indeed, it's is possible to dump the registers with
ethtool.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>