dmaengine: stm32-dma: Fix typo in Kconfig

As STM32 DMA driver is only used as buit-in driver, it couldn't be used as
module.

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Reviewed-by: Ludovic BARRE <ludovic.barre@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>