drivers: serial: stm32U5 uart driver do not toggle the DMA Tx request

Errata sheet of the stm32U5 (ES0499) recommends to avoid clearing
the DMAT bit with LL_USART_DisableDMAReq_TX to re-start
a DMA transfer on the UART Tx block. The function becomes empty.
This is also seen for stm32H5.

Signed-off-by: Francois Ramu <francois.ramu@st.com>