mmc: sdhci-esdhc: Make 8BIT bus work

Support for 8BIT bus with was added some time ago to sdhci-esdhc but
then missed to remove the 8BIT from the reserved bit mask which made
8BIT non functional.

Fixes: 66b50a00 ("mmc: esdhc: Add support for 8-bit bus width and..")
Signed-off-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>