gst_omx_port_set_flushing: simplify waiting loop
gst_omx_port_set_flushing: simplify waiting loop

No semantic change so far, I just made the 'while' end condition easier
to understand as a first step before changing it.
- move error/time out checks inside the loop to make it clearer on what
we are actually waiting for.
- group port->buffers checks together with parenthesis as they are part
of the same conceptual check: waiting for all buffers to be released.

https://bugzilla.gnome.org/show_bug.cgi?id=789475

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx