mfd: STw481x: Allow modular build

This driver depends on I2C, which may be a loadable module.
While you'd probably want both to be built-in in practice,
allowing a modular build avoids possible randconfig link
errors.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -1225,7 +1225,7 @@ config MFD_WM8994
 	  functionaltiy of the device other drivers must be enabled.
 
 config MFD_STW481X
-	bool "Support for ST Microelectronics STw481x"
+	tristate "Support for ST Microelectronics STw481x"
 	depends on I2C && ARCH_NOMADIK
 	select REGMAP_I2C
 	select MFD_CORE