bitrate: fix calculation to match API description

bitrate: fix calculation to match API description

rte_stats_bitrate_calc() API states it returns 'Negative value on error'.

However, the implementation will return the error code from
rte_eth_stats_get() which may be non-zero on error.

Change the implementation of rte_stats_bitrate_calc() to match
the API description by always returning a negative value on error.

Fixes: 2ad7ba9a6567 ("bitrate: add bitrate statistics library")
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>