stmmac: fix the rx csum feature

For new GMACs it is possible to turn-on/off the COE.
In the current driver, when disabled the Rx-checksum
via ethtool, the tool reported that csum was disabled
but the HW continued to set the IPC. Indeed this is
because the fix_features allows this. So the patch
fixes this problem by adding the set_features.

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