drivers: dma_stm32: init DMA struct in dma_stm32_configure

The init struct for DMA peripheral is allocated in stack
and must be initialized (e.g. with LL_DMA_StructInit here)
to avoid loading undefined values in the DMA peripheral
registers.

Signed-off-by: Abel Radenac <a.radenac@lacroix-sofrel.com>