net: hisilicon: hns: fix error return code of hns_nic_clear_all_rx_fetch()

When hns_assemble_skb() returns NULL to skb, no error return code of
hns_nic_clear_all_rx_fetch() is assigned.
To fix this bug, ret is assigned with -ENOMEM in this case.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>