stmmac: dwmac-sti: remove unused priv dev member

The dev member of struct sti_dwmac is not used anywhere in the driver
so lets just remove it.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>