net: stmmac: make stmmac_mdio_reset non-static

This patch promotes stmmac_mdio_reset function from static to
non-static, so that power management functions can decide to reset if
the IP comes out from lowe power state specially hibernation cases.

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