net: Add net_update_ether() to handle ARP and Ping replies

When the network is VLAN or SNAP, net_update_ether() will preserve
the original Ethernet packet header and simply replace the src and
dest MACs and the protocol

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>