iio_buffer_push(): Fix return value in high-speed mode
iio_buffer_push(): Fix return value in high-speed mode

The get_buffer() callback on success returns the number of bytes used in
the freshly dequeued buffer. Whereas the iio_buffer_push() function is
supposed to return the number of bytes in the buffer that was just queued.

Signed-off-by: Lars-Peter Clausen <3318dc5ce3e4fb7c28a0b841b6801c884e1d0896@metafoo.de>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio