net: stmmac: xgmac: Clear previous RX buffer size

When switching between buffer sizes we need to clear the previous value.

Fixes: d6ddfacd ("net: stmmac: Add DMA related callbacks for XGMAC2")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>