ethdev: add return value to stats get dev op

ethdev: add return value to stats get dev op

The stats_get dev op API doesn't include return value, so PMD cannot
return an error in case of failure at stats getting process time.

Since PCI devices can be removed and there is a time between the
physical removal to the RMV interrupt, the user may get invalid stats
without any indication.

This patch changes the stats_get API return value to be int instead of
void.

All the net PMDs stats_get dev ops are adjusted by this patch.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>