stmmac: tidy-up stmmac_priv structure

This patch tidies-up the stmmac_priv structure
that had many fileds alredy defined in the
plat_stmmacenet_data structure.

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