net: l2: ethernet: Fix error handling after ARP prepare

Commit 55802e5e fixed error handling of
TX errors, in case ARP request was generated. There are however also
other places where post-ARP cleanup should be done on the TX path (like
running out of buffers for Ethernet L2 header allocation).

This commit fixes those cases in ethernet_send(), where function would
exit early and report error after ARP prepare stage.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>