stmmac: dwmac-sti: clean up and rename sti_dwmac_init

Rename sti_dwmac_init to sti_dwmac_set_mode which is a better
description for what it really does.

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>