net: stmmac: fix some code style problem

Checkpatch complains about some code style problem on stmmac_mdio.c.
This patch fix them.

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>