net: stmmac: Only the last buffer has the FCS field

Only the last received buffer contains the FCS field. Check for end of
packet before trying to strip the FCS field.

Fixes: 88ebe2cf ("net: stmmac: Rework stmmac_rx()")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>