mfd: Make MFD_AS3722 depend on I2C=y

MFD_AS3722 can only be builtin, so it needs I2C builtin as well.
With I2C=m, we get:

drivers/mfd/as3722.c:372: undefined reference to `devm_regmap_init_i2c'
drivers/built-in.o: In function `as3722_i2c_driver_init':
drivers/mfd/as3722.c:444: undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `as3722_i2c_driver_exit':
drivers/mfd/as3722.c:444: undefined reference to `i2c_del_driver'
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -32,7 +32,7 @@ config MFD_AS3722
 	select MFD_CORE
 	select REGMAP_I2C
 	select REGMAP_IRQ
-	depends on I2C && OF
+	depends on I2C=y && OF
 	help
 	  The ams AS3722 is a compact system PMU suitable for mobile phones,
 	  tablets etc. It has 4 DC/DC step-down regulators, 3 DC/DC step-down