stmmac: fix chained mode

This patch is to fix the chain mode that was broken
and generated a panic. This patch reviews the chain/ring
modes now shaing the same structure and taking care
about the pointers and callbacks.

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