mmc: sh_mmcif: Use sg_miter for PIO

mmc: sh_mmcif: Use sg_miter for PIO

Use sg_miter iterator instead of sg_virt() and custom code
to loop over the scatterlist. The memory iterator will do
bounce buffering if the page happens to be located in high memory,
which the driver may or may not be using.

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-v2-9-d8e732aa97d1@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>