mfd: max14577: Allow driver to be built as a module

The driver's Kconfig symbol is a boolean but nothing prevents the driver
to be built as a module instead of built-in. It is true that most system
integrators will choose the latter but the config should not restrict it.
Suggested-by: NKrzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: NJavier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -535,8 +535,8 @@ config MFD_88PM860X
 	  battery-charger under the corresponding menus.
 
 config MFD_MAX14577
-	bool "Maxim Semiconductor MAX14577/77836 MUIC + Charger Support"
-	depends on I2C=y
+	tristate "Maxim Semiconductor MAX14577/77836 MUIC + Charger Support"
+	depends on I2C
 	select MFD_CORE
 	select REGMAP_I2C
 	select REGMAP_IRQ