mmc: renesas_sdhi: make setup selection more understandable

When I read 'no_fallback', I forgot what fallback even though I was the
author of this change. Name it better to make the code easier to
understand.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20220320123016.57991-4-wsa+renesas@sang-engineering.com


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