mfd: 88pm860x-i2c: Fix variable length array Sparse warning

drivers/mfd/88pm860x-i2c.c:125:33:
  warning: Variable length array is used.
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/88pm860x-i2c.c

@@ -122,7 +122,7 @@ static int read_device(struct i2c_client *i2c, int reg,
 static int write_device(struct i2c_client *i2c, int reg,
 			int bytes, void *src)
 {
-	unsigned char buf[bytes + 1];
+	unsigned char buf[2];
 	struct i2c_adapter *adap = i2c->adapter;
 	struct i2c_msg msg;
 	int ret;