ethdev: make stats and xstats reset callbacks return int

ethdev: make stats and xstats reset callbacks return int

Change return value of the callbacks from void to int. Make
implementations across all drivers return negative errno
values in case of error conditions.

Both callbacks are updated together because a large number of
drivers assign the same function to both callbacks.

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>