dmaengine: stm32-dma: make stm32_dma_set_config static

Sparse complains:
drivers/dma/stm32-dma.c:957:6: warning: symbol 'stm32_dma_set_config' was
not declared. Should it be static?

SO make stm32_dma_set_config static.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>