mmc: sdhci: change CONFIG of MMC_SDHCI_TEGRA from 'tristate' to 'bool'

This config option is used to decide whether tegra sdhci support
should be build into the sdhci-platform module.  So setting it 'm'
is nonsense, since we can't build part of an module as a module.
The effect for 'm' will probably be the same as for 'n'.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>