mmc: atmel-mci: fix incorrect setting of host->data to NULL

Setting host->data to NULL is incorrect sequence in STATE_SENDING_STOP
state of FSM: This early setting leads to the skip of dma_unmap_sg()
in atmci_dma_cleanup() which is a bug.

Idea taken from dw_mmc by Seungwon Jeon.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Seungwon Jeon <tgih.jun@samsung.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>