iio_device_debug_attr_write_*: fix return value
iio_device_debug_attr_write_*: fix return value

make these functions return 0 on success instead of the number of
bytes written, in accordance with the API

gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,sensarliar/libiio,sensarliar/libiio,Sunderfield/libiio,Sunderfield/libiio,Sunderfield/libiio,gburca/libiio,sensarliar/libiio,gburca/libiio,analogdevicesinc/libiio