mmc: dw_mmc: Regression fix for non-IDMAC DMA

3ec7699d3bb1b0ee7 ("mmc: dw_mmc: Add support for pre_req and post_req")
broke non-IDMAC DMA, because dw_mci_pre_dma_transfer() is valid only if
using internal DMA. In case of using other DMA it returns -ENOSYS. It
prevents the DMA operations.  This patch makes dw_mci_pre_dma_transfer()
effective in all DMA cases again.

Reported-by: James Hogan <james@albanarts.com>
Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>