Move the setting of mmc->actual_clock to zero into the set_clock
handlers themselves.  This will allow us to clean up the calling
logic for the set_clock() method, and turn sdhci_set_clock() into
a library function.
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>