net: stmmac: Fix bad RX timestamp extraction

[ Upstream commit a1762456993893795030d911106a7650481db0ef ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>