phy: Constify struct phy_ops variables

The phy_ops variables are never modified after initialized in these
drivers, so make them const.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>