net: stmmac: xgmac: Enable RX Jumbo frame support

We are already doing it by default in the TX path so we can also enable
Jumbo Frame support in the RX path independently of MTU value.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>