liquidio: Fix wrong return value in cn23xx_get_pf_num()

On an error exit path, a negative error code should be returned
instead of a positive return value.

Fixes: 0c45d7fe ("liquidio: fix use of pf in pass-through mode in a virtual machine")
Cc: Rick Farrington <ricardo.farrington@cavium.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>