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.