stm32cube: stm32f7xx: drivers: 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 DuESC0 register.
This fix allows to effectively set the TX timestamp properly when there
is one.

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