dmaengine: stm32-dma: remove unused ‘sfcr’

In stm32_dma_chan_irq(), variable ‘sfcr’ is initialized but never used, which
leads to warning with W=1

drivers/dma/stm32-dma.c: In function ‘stm32_dma_chan_irq’:
drivers/dma/stm32-dma.c:530:19: warning: variable ‘sfcr’ set but not used [-Wunused-but-set-variable]
  u32 status, scr, sfcr;

So remove it.

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