mmc: mmci: Add small comment about reset thread

Put a small comment before assigning IRQ_WAKE_THREAD
telling us what is going on.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Yann Gautier <yann.gautier@foss.st.com>
Cc: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210921143359.1738149-1-linus.walleij@linaro.org


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