net: ethernet: Clean an obscure ref/unref logic in ethernet_send()

Do not extra ref and then obscurely unref the packet inside
the statistics update function.

Actually, this extra ref/unref isn't needed here at all.

The packet is unreferenced only on a successful send, statistics
updating can be done before the unref in a clean and understandable way.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>