net: stmmac: unify registers dumps methods

The stmmac driver have two methods for registers dumps: via ethtool and
at init (if NETIF_MSG_HW is enabled).

It is better to keep only one method, ethtool, since the other was ugly.

This patch convert all dump_regs() function from "printing regs" to
"fill the reg_space used by ethtool".

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>