usnic: correctly check failed allocation

Since ib_alloc_device returns allocated memory address, not error,
it should be checked as IS_NULL, not IS_ERR_OR_NULL.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Reviewed-by: Dave Goodell <dgoodell@cisco.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>