IIOD: Fix write_dev_attr always returning success

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added API functions to read/write debug attributes by type

Those functions are:

iio_device_debug_attr_read_bool
iio_device_debug_attr_read_longlong
iio_device_debug_attr_read_double

iio_device_debug_attr_write_bool
iio_device_debug_attr_write_longlong
iio_device_debug_attr_write_double

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>