mmc: tmio: remove outdated comment

The obviously wrong comment was added in 2011 with commit df3ef2d3
("mmc: protect the tmio_mmc driver against a theoretical race") but
already obsoleted half a year later with commit b9269fdd ("mmc:
tmio: fix recursive spinlock, don't schedule with interrupts disabled").

Fixes: b9269fdd ("mmc: tmio: fix recursive spinlock, don't schedule with interrupts disabled")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>