dmaengine: fsldma: remove the unused variable

Fix the following build warning:
  drivers/dma/fsldma.c: In function 'fsl_dma_device_terminate_all':
  drivers/dma/fsldma.c:947:6: warning: unused variable 'size' [-Wunused-variable]

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>