nfp: abm: fix error return code in nfp_abm_vnic_alloc()

Fix to return negative error code -ENOMEM from the kzalloc() error
handling case instead of 0, as done elsewhere in this function.

Fixes: 174ab544 ("nfp: abm: add cls_u32 offload for simple band classification")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>