mmc: sdhci-iproc: Disable preset values for BCM2835

According to the BCM2835 datasheet there are no preset value registers.
This wasn't an issue before, because we didn't propagate 1.8V support.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>