mmc: core Convert UNSTUFF_BITS macro to inline function

The UNSTUFF_BITS macro, which is defined in both drivers/mmc/core/mmc.c
and drivers/mmc/core/sd.c, has been converted to an inline function to
improve readability, maintainability, and type safety.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Link: https://lore.kernel.org/r/20240902123331.3566447-1-avri.altman@wdc.com


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