stmmac: allow mmc usage only if feature actually available (V4)

Enable the MMC support if it is actually available from the
HW capability register.

Signed-off-by: Rayagond Kokatanur <rayagond@vayavyalabs.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>