s390/ism: fix error return code in ism_probe()

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

Fixes: 684b89bc ("s390/ism: add device driver for internal shared memory")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>