ext/wavpack/: Some small clean-ups: use enums instead of hard-coded numbers, const...

ext/wavpack/: Some small clean-ups: use enums instead of hard-coded numbers, const-ify element details, re-factor som...

Original commit message from CVS:
Patch by: Sebastian Dröge  <slomo@circular-chaos.org>
* ext/wavpack/gstwavpackdec.c: (gst_wavpack_dec_reset),
(gst_wavpack_dec_init), (gst_wavpack_dec_change_state):
* ext/wavpack/gstwavpackenc.c: (gst_wavpack_enc_base_init),
(gst_wavpack_enc_class_init), (gst_wavpack_enc_reset),
(gst_wavpack_enc_init), (gst_wavpack_enc_set_wp_config),
(gst_wavpack_enc_change_state):
* ext/wavpack/gstwavpackparse.c:
Some small clean-ups: use enums instead of hard-coded numbers,
const-ify element details, re-factor some code into _reset()
functions (#352605).