platform/x86: msi-wmi: Fix variable 'status' set but not used compiler warning

Explicitly check the status rather then relying on output.pointer staying
NULL on an error.

Reported-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210204121931.131617-1-hdegoede@redhat.com