usnic: correctly handle kzalloc return value

Since kzalloc returns memory address, not error code,
it should be checked whether it is null or not.

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