net: ethernet: arp: Fix incorrect adding to the pending queue

k_fifo_put is a macro that expands the call to net_pkt_ref(pending)
multiple times when CONFIG_TRACING is enabled thus causing extra
reference for the pending packet and a memory leak.
Fix by moving the referencing call to a separate line.
Signed-off-by: Rait Rääk <raitraak@gmail.com>