net: ethernet: mediatek: fix a typo bug in flow offloading

Issue was traffic problems after a while with increased ping times if
flow offload is active. It turns out that key_offset with cookie is
needed in rhashtable_params but was re-assigned to head_offset.
Fix the assignment.

Fixes: 502e84e2 ("net: ethernet: mtk_eth_soc: add flow offloading support")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: David S. Miller <davem@davemloft.net>