stm32f7: fix TX timestamp check

It looks like the original code was a bad copy-paste from H5/H7 HALs.
ETH_DMATXDESC_LS and ETH_DMATXDESC_TTSS are flags from the DESC0 register.
This fix allows to effectively set the TX timestamp properly when there
is one.

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

Src/stm32f7xx_hal_eth.c

diff --git a/Src/stm32f7xx_hal_eth.c b/Src/stm32f7xx_hal_eth.c
index 77d6f84..3994e5b 100644
--- a/Src/stm32f7xx_hal_eth.c
+++ b/Src/stm32f7xx_hal_eth.c
@@ -1472,8 +1472,8 @@
       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;