1. fix error return type in flex_button_register
[update]
1. add return type of flex_button_process.
2. add return type of flex_button_scan.
In low power system,the button task can get the return value when call flex_button_scan.Task stop polling when return 0,otherwise go on polling.In this case,the interrupt of button should be enabled,mcu will be waked up when get interrupt,and polling will restart.
Signed-off-by: wzd5230 <772461552@qq.com>
