mmc: sdhci: remove comment regarding timeout during tuning

Since commit 7ce45e95 ("mmc: sdhci: SD tuning is broken for some
controllers") sdhci_execute_tuning() no longer includes a timeout in its
loop counter(s) so remove portion of the comment regarding this.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>