mfd: Don't allow wm831x to be built as a module

The genirq infrastructure functions aren't currently exported,
preventing modular builds.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/Kconfig

@@ -195,7 +195,7 @@ config MFD_WM8400
 	  the functionality of the device.
 
 config MFD_WM831X
-	tristate "Support Wolfson Microelectronics WM831x/2x PMICs"
+	bool "Support Wolfson Microelectronics WM831x/2x PMICs"
 	select MFD_CORE
 	depends on I2C
 	help