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 <1f46c93815774a2e602547cf1e37c682d62b566d@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk