stmmac: only remove RXCSUM feature if no rx coe is available

In case of the HW is not able to do the receive checksum offloading
the only feature to remove is NETIF_F_RXCSUM.

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