net: stmmac: Fix bad RX timestamp extraction

As noted in dwmac4_wrback_get_rx_timestamp_status the timestamp is found
in the context descriptor following the current descriptor. However the
current code looks for the context descriptor in the current
descriptor, which will always fail.

Signed-off-by: Fredrik Hallenberg <megahallon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>