mmc: sdhci: avoid double-delay while transitioning to 1.8V

The MMC core in mmc_set_signal_voltage() already provides for the delay
required to switch to 1.8V, so there is no need for drivers to perform
this wait themselves.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>