drm/xe: Fix warning on impossible condition

Having a different value for op is not possible: this is already kept
out of user-visible warning by the check in xe_wait_user_fence_ioctl()
if op > MAX_OP. The warning is useful as if this switch() is not update
when a new op is added, it should be triggered.

Fix warning as reported by 0-DAY CI Kernel:

	drivers/gpu/drm/xe/xe_wait_user_fence.c:46:2: warning: variable 'passed' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]

Closes: https://lore.kernel.org/oe-kbuild-all/202312170357.KPSinwPs-lkp@intel.com/
Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20231218163301.3453285-1-lucas.demarchi@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_wait_user_fence.c

@@ -46,6 +46,7 @@ static int do_compare(u64 addr, u64 value, u64 mask, u16 op)
 		break;
 	default:
 		XE_WARN_ON("Not possible");
+		return -EINVAL;
 	}
 
 	return passed ? 0 : 1;