stm32h7: fix TX timestamp check

It looks like the original code was a bad copy-paste from H5 HAL.
ETH_DMATXDESC_LS and ETH_DMATXDESC_TTSS are flags from the DESC0 register.
This allows to effectively et the TX timestamp properly when there is one.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

stm32cube/stm32f7xx/drivers/src/stm32f7xx_hal_eth.c

@@ -1472,8 +1472,8 @@ HAL_StatusTypeDef HAL_ETH_ReleaseTxPacket(ETH_HandleTypeDef *heth)
       if ((heth->Init.TxDesc[idx].DESC0 & ETH_DMATXDESC_OWN) == 0U)
       {
 #ifdef HAL_ETH_USE_PTP
-        if ((heth->Init.TxDesc[idx].DESC3 & ETH_DMATXDESC_LS)
-            && (heth->Init.TxDesc[idx].DESC3 & ETH_DMATXDESC_TTSS))
+        if ((heth->Init.TxDesc[idx].DESC0 & ETH_DMATXDESC_LS)
+            && (heth->Init.TxDesc[idx].DESC0 & ETH_DMATXDESC_TTSS))
         {
           /* Get timestamp low */
           timestamp->TimeStampLow = heth->Init.TxDesc[idx].DESC6;