voamrwbenc: fix list of bitrates
The list of #defines representing the different AMR-WB encoding "modes" (= bitrates) was incorrect from 14.25 kbps onwards, as value 2 appeared twice in the list. This means that configuring the element to encode at e.g. 23.85 kbps (using the "band-mode" property) resulted in 23.05 kbps being used instead, and so on for other bitrates except the lowest few.