net: stmmac: RX Descriptors need to be clean before setting buffers

RX Descriptors are being cleaned after setting the buffers which may
lead to buffer addresses being wiped out.

Fix this by clearing earlier the RX Descriptors.

Fixes: 2af6106a ("net: stmmac: Introducing support for Page Pool")
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>