net: stmmac: Add missing call to dev_kfree_skb()

When RX HW timestamp is enabled and a frame is discarded we are
not freeing the skb but instead only setting to NULL the entry.

Add a call to dev_kfree_skb_any() so that skb entry is correctly
freed.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>