mmc: sdhci: clean up sdhci_update_clock()/sdhci_set_clock()

Only one caller to sdhci_set_clock() needs to check whether the
requested clock frequency was the same as the currently set frequency,
yet we work around this in several other sites via sdhci_update_clock().
Rather than doing this, move those checks out into sdhci_do_set_ios(),
which then allows sdhci_update_clock() to be eliminated.

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>