mmc: moxart-mmc: Factor out moxart_use_dma() helper

The same code is in two places and we will add a third place.
Break this out into its own function.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240127-mmc-proper-kmap-v2-2-d8e732aa97d1@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>