mmc: sdhci: clean up sdhci_execute_tuning() decision

Clean up the code in sdhci_execute_tuning() so the decision whether
to execute tuning is clearer - and despite this reflecting what the
original code was doing, it shows that it may not be what the author
actually intended.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>