mmc: moxart-mmc: Fix accounting in DMA transfer

The whole scatterlist chain is submitted to the DMA engine,
but the code is written to just account for the length of
the first sg entry.

When the DMA transfer is finished, all the data in the
request has been transferred, account for this instead.

This only works because the moxart_request() function isn't
checking that all data was transferred and will
unconditionally issue mmc_request_done() after returning
successfully from moxart_transfer_dma().

Keep the assignment of accounted bytes in .bytes_xfered
but move it after the completion where we know it has
actually happened.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240127-mmc-proper-kmap-v2-3-d8e732aa97d1@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>