net: stmmac: Enable 16KB buffer size

commit b2f3a481c4cd62f78391b836b64c0a6e72b503d2 upstream.

XGMAC supports maximum MTU that can go to 16KB. Lets add this check in
the calculation of RX buffer size.

Fixes: 7ac6653a ("stmmac: Move the STMicroelectronics driver")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>