mmc: dw_mmc: prevent NULL dereference for dma_ops

Now, dma_ops is assumed that use the IDMAC.  But if dma_ops is assigned
the pdata->dma_ops, we didn't ensure that callback function is defined.

If the callback isn't defined, then we should run in PIO mode.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>