[stm32] Fix reconfiguring DMAMUX requests

Configuring a DMAMUX request was only working once because the bits
were not properly cleared.

Author: chris-durand

src/modm/platform/dma/stm32/dma.hpp.in

@@ -295,7 +295,7 @@ public:
 %% endif
 			})->muxChannel;
 			auto* channel = DMAMUX1_Channel0 + muxChannel;
-			channel->CCR = (channel->CCR & DMAMUX_CxCR_DMAREQ_ID) | uint32_t(dmaRequest);
+			channel->CCR = (channel->CCR & ~DMAMUX_CxCR_DMAREQ_ID) | uint32_t(dmaRequest);
 %% elif dmaType in ["stm32-stream-channel"]
 			DMA_Channel_TypeDef *Channel = reinterpret_cast<DMA_Channel_TypeDef*>(CHANNEL_BASE);
 			Channel->CR = (Channel->CR & ~DMA_SxCR_CHSEL_Msk) | uint32_t(dmaRequest);