ibmvnic: fix error return code in ibmvnic_probe()

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

Fixes: 032c5e82 ("Driver for IBM System i/p VNIC protocol")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>