platform/surface: surface3-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. This silences the following compiler warning:

drivers/platform/surface/surface3-wmi.c:60:14: warning: variable 'status' set but not used [-Wunused-but-set-variable]

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210204113848.105994-1-hdegoede@redhat.com