ethdev: change xstats reset function to return int

ethdev: change xstats reset function to return int

Change rte_eth_xstats_reset() return value from void to int and
return negative errno values in case of error conditions.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>