mmc: dw_mmc: fix incorrect setting of host->data of NULL

Setting host->data to NULL is incorrect sequence in
dw_mci_command_complete. This early setting makes the skip of
dma_unmap_sg in dw_mci_dma_cleanup.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>