ethdev: add return code to stats reset function

Some devices do not support reset of eth stats.  An application may
need to know not to clear shadow stats if the device cannot.

rte_eth_stats_reset is updated to provide a return code to share
whether the device supports reset or not.

Signed-off-by: David Harton <dharton@cisco.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>