flvmux: Correct breaks in gst_flv_mux_find_best_pad

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.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/710>