mfd: Don't support non-modular wm8400 build

It's relying on non-exported symbols.
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

@@ -465,7 +465,7 @@ config MFD_S5M_CORE
 	 of the device
 
 config MFD_WM8400
-	tristate "Support Wolfson Microelectronics WM8400"
+	bool "Support Wolfson Microelectronics WM8400"
 	select MFD_CORE
 	depends on I2C
 	select REGMAP_I2C