IB: Fix return value in ib_device_register_sysfs()

If kobject_create_and_add() fails and returns NULL, the current code
in ib_device_register_sysfs() does not set ret and hence returns 0.
Set ret to -ENOMEM for this failure, so that the caller knows that
ib_device_register_sysfs() actually failed.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>