octeontx2-pf: Fix error return code in otx2_probe()

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

Fixes: 5a6d7c9d ("octeontx2-pf: Mailbox communication with AF")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>