flvmux: Correct breaks in gst_flv_mux_find_best_pad

The code seems to use continue and break as if both refer to the surrounding while loop. But because break breaks out of the switch, they actually have the same effect.

This may have caused the loop not to terminate when it should. E.g. when skip_backwards_streams drops a buffer we should abort the aggregation and wait for all pads to be filled again. Instead, we might have just selected a subsequent pad as our new "best".

Replace break with done = TRUE; break, and continue with break. Then simplify the code a bit.