bitrate: fix registration to match API description

bitrate: fix registration to match API description

rte_stats_bitrate_reg() API states it returns 'Zero on success'.

However, the implementation directly returns the return of
rte_metrics_reg_names() which may be zero or positive on success,
with a positive value also indicating the index.

The user of rte_stats_bitrate_reg() should not care about the
index as it is stored in the opaque rte_stats_bitrates struct.

Change the implementation of rte_stats_bitrate_reg() to match
the API description by always returning zero on success.

Fixes: 2ad7ba9a6567 ("bitrate: add bitrate statistics library")

Signed-off-by: Kevin Traynor <ktraynor@redhat.com>