mmc: tmio: comment the ERR_PTR usage in this driver

It is not super obvious why the driver sometimes uses an ERR_PTR for the
current mrq. Explain why in comments.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240305105623.3379-2-wsa+renesas@sang-engineering.com


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