misc: bcm2835_smi: Use proper enum types for dma_{,un}map_single()

Clang warns:

  drivers/misc/bcm2835_smi.c:692:4: warning: implicit conversion from enumeration type 'enum dma_transfer_direction' to different enumeration type 'enum dma_data_direction' [-Wenum-conversion]
                          DMA_MEM_TO_DEV);
                          ^~~~~~~~~~~~~~~
  ./include/linux/dma-mapping.h:406:66: note: expanded from macro 'dma_map_single'
  #define dma_map_single(d, a, s, r) dma_map_single_attrs(d, a, s, r, 0)
                                     ~~~~~~~~~~~~~~~~~~~~          ^
  drivers/misc/bcm2835_smi.c:705:35: warning: implicit conversion from enumeration type 'enum dma_transfer_direction' to different enumeration type 'enum dma_data_direction' [-Wenum-conversion]
                          (inst->dev, phy_addr, n_bytes, DMA_MEM_TO_DEV);
                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
  ./include/linux/dma-mapping.h:407:70: note: expanded from macro 'dma_unmap_single'
  #define dma_unmap_single(d, a, s, r) dma_unmap_single_attrs(d, a, s, r, 0)
                                       ~~~~~~~~~~~~~~~~~~~~~~          ^
  drivers/misc/bcm2835_smi.c:751:12: warning: implicit conversion from enumeration type 'enum dma_transfer_direction' to different enumeration type 'enum dma_data_direction' [-Wenum-conversion]
                                                       DMA_DEV_TO_MEM);
                                                       ^~~~~~~~~~~~~~~
  ./include/linux/dma-mapping.h:406:66: note: expanded from macro 'dma_map_single'
  #define dma_map_single(d, a, s, r) dma_map_single_attrs(d, a, s, r, 0)
                                     ~~~~~~~~~~~~~~~~~~~~          ^
  drivers/misc/bcm2835_smi.c:761:50: warning: implicit conversion from enumeration type 'enum dma_transfer_direction' to different enumeration type 'enum dma_data_direction' [-Wenum-conversion]
                  dma_unmap_single(inst->dev, phy_addr, n_bytes, DMA_DEV_TO_MEM);
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
  ./include/linux/dma-mapping.h:407:70: note: expanded from macro 'dma_unmap_single'
  #define dma_unmap_single(d, a, s, r) dma_unmap_single_attrs(d, a, s, r, 0)
                                       ~~~~~~~~~~~~~~~~~~~~~~          ^
  4 warnings generated.

Use the proper enumerated type to clear up the warning. There is not
actually a bug here because the enumerated types have the same integer
value:

DMA_MEM_TO_DEV = DMA_TO_DEVICE = 1
DMA_DEV_TO_MEM = DMA_FROM_DEVICE = 2

Fixes: 93254d0f ("Add SMI driver")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>