Local backend: Fix timeouts in streaming mode with low-speed devices

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Make sure that iio_buffer_push sends the full buffer to the device

Previously, when calling iio_buffer_push frequently on a low-speed
device, the buffers were only partly uploaded.

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