mmc: mvsdio: Use sg_miter for PIO

Use the scatterlist memory iterator instead of just
dereferencing virtual memory using sg_virt().
This make highmem references work properly.

This driver also has a bug in the PIO sglist handling that
is fixed as part of the patch: it does not travers the
list of scatterbuffers: it will just process the first
item in the list. This is fixed by augmenting the logic
such that we do not process more than one sgitem
per IRQ instead of counting down potentially the whole
length of the request.

We can suspect that the PIO path is quite untested.

Suggested-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/linux-mmc/20240122073423.GA25859@lst.de/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240127-mmc-proper-kmap-v2-5-d8e732aa97d1@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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