mmc: omap_hsmmc: fix oops in omap_hsmmc_dma_cb()

In the case of an I/O error, the DMA will have been cleaned up in
the MMC interrupt and the request structure pointer will be null.

In that case, it is essential to check if the DMA is over before
dereferencing host->mrq->data.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>