staging: bcm2708_vchiq: fix return value check in vchiq_platform_conn_state_changed()

In case of error, the function kthread_create() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -2774,7 +2774,7 @@ void vchiq_platform_conn_state_changed(VCHIQ_STATE_T *state,
 				&vchiq_keepalive_thread_func,
 				(void *)state,
 				threadname);
-			if (arm_state->ka_thread == NULL) {
+			if (IS_ERR(arm_state->ka_thread)) {
 				vchiq_log_error(vchiq_susp_log_level,
 					"vchiq: FATAL: couldn't create thread %s",
 					threadname);