e1000e: Don't return uninitialized stats

Some statistics passed to ethtool are garbage because e1000e_get_stats64()
doesn't write them, for example: tx_heartbeat_errors. This leaks kernel
memory to userspace and confuses users.

Do like ixgbe and use dev_get_stats() which first zeroes out
rtnl_link_stats64.

Fixes: 5944701d ("net: remove useless memset's in drivers get_stats64")
Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>