drivers: serial: stm32 uart defines dma slot with macro

The STM32_DMA_SLOT macro from include/drivers/dma/dma_stm32.h
must be used here, especially for dma of type v2bis.
In this case, the dma-cell is not defined and slot is null.

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