net: stmmac: use correct define to get rx timestamp on GMAC4

In dwmac4_wrback_get_rx_timestamp_status we looking for a RX timestamp.
For that receive descriptors are handled and so we should use defines
related to receive descriptors. It'll no change the functional behavior
as RDES3_RDES1_VALID=TDES3_RS1V=BIT(26) but it makes code easier to read.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>