mmc: sdhci: move setting host->clock into sdhci_do_set_ios()

We don't need implementations to do this, since the only time it's
necessary is when we change the clock, and the only place that happens
is in sdhci_do_set_ios().  So, move it there, and remove it from the
iMX platform backend.

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>