mmc: debugfs: add description for module parameter

Make it obvious what this is for.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210604104459.7574-1-wsa+renesas@sang-engineering.com


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