acpi: Replace weird use of PTR_RET.

This functions is really weird.  It sets rc to -ENOMEM, then overrides
it.  It was converted to PTR_RET in a1458187 when it should have
simply been rewritten.

This version makes it more explicit, with a single IS_ERR() test.

Cc: Alexandru Gheorghiu <gheorghiuandru@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_pad.c

@@ -231,16 +231,19 @@ static struct task_struct *ps_tsks[NR_CPUS];
 static unsigned int ps_tsk_num;
 static int create_power_saving_task(void)
 {
-	int rc = -ENOMEM;
+	int rc;
 
 	ps_tsks[ps_tsk_num] = kthread_run(power_saving_thread,
 		(void *)(unsigned long)ps_tsk_num,
 		"acpi_pad/%d", ps_tsk_num);
-	rc = PTR_RET(ps_tsks[ps_tsk_num]);
-	if (!rc)
-		ps_tsk_num++;
-	else
+
+	if (IS_ERR(ps_tsks[ps_tsk_num])) {
+		rc = PTR_ERR(ps_tsks[ps_tsk_num]);
 		ps_tsks[ps_tsk_num] = NULL;
+	} else {
+		rc = 0;
+		ps_tsk_num++;
+	}
 
 	return rc;
 }