staging: vchiq_arm: Drop unnecessary NULL check

Looking at the handling of service instance within the VCHIQ
driver shows that it's not possible that service_callback is
called with instance is a NULL pointer. So drop the unnecessary
NULL check and fix:

vchiq_arm.c:1109 service_callback() warn:
  variable dereferenced before check 'instance' (see line 1091)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/linux-arm-kernel/202404230315.vx7ESZ3r-lkp@intel.com/
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Link: https://lore.kernel.org/r/20240425165852.6681-2-wahrenst@gmx.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c

@@ -1106,7 +1106,7 @@ service_callback(struct vchiq_instance *instance, enum vchiq_reason reason,
 
 	user_service = (struct user_service *)service->base.userdata;
 
-	if (!instance || instance->closing) {
+	if (instance->closing) {
 		rcu_read_unlock();
 		return 0;
 	}