mmc: sdhci: set_uhs_signaling() need not return a value

The set_uhs_signaling() method gives the impression that it can fail,
but anything returned from the method is entirely ignored by the sdhci
driver.  So returning failure has no effect.

So, kill the idea that it's possible for this to return an error by
removing the returned value.

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>