iio: adc: Nothing in ADC should be a bool CONFIG

The whole IIO subsystem can be built as modules.  If you make it a
module then stuff marked as "Y" in the adc directory simply won't be
linked in properly.

The two configs that were wrong were EXYNOS_ADC and LP8788_ADC.  I
know for a fact that EXYNOS_ADC will work as a module. I assume
LP8788_ADC will also be fine.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -106,15 +106,15 @@ config AT91_ADC
 	  Say yes here to build support for Atmel AT91 ADC.
 
 config EXYNOS_ADC
-	bool "Exynos ADC driver support"
+	tristate "Exynos ADC driver support"
 	depends on OF
 	help
 	  Core support for the ADC block found in the Samsung EXYNOS series
 	  of SoCs for drivers such as the touchscreen and hwmon to use to share
 	  this resource.
 
 config LP8788_ADC
-	bool "LP8788 ADC driver"
+	tristate "LP8788 ADC driver"
 	depends on MFD_LP8788
 	help
 	  Say yes here to build support for TI LP8788 ADC.