stmmac: dity-up and rework the driver debug levels

Prior this patch, the internal debugging was based on ifdef
and also some printk were useless because many info are exposed
via ethtool.
This patch remove all the ifdef defines and now we only use
netif_msg_XXX levels.

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