stmmac: prefetch right address

commit 4744bf07 upstream.

To support XDP, a headroom is prepended to the packet data.
Consider this offset when doing a prefetch.

Fixes: da5ec7f2 ("net: stmmac: refactor stmmac_init_rx_buffers for stmmac_reinit_rx_buffers")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>