powerpc: sysdev: use lock guard for mutex

use guard(mutex) for scope based resource management of mutex
This would make the code simpler and easier to maintain.

More details on lock guards can be found at
https://lore.kernel.org/all/20230612093537.614161713@infradead.org/T/#u

Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250505075333.184463-7-sshegde@linux.ibm.com

Author: Shrikanth Hegde

arch/powerpc/sysdev/fsl_mpic_timer_wakeup.c

@@ -75,39 +75,31 @@ static ssize_t fsl_timer_wakeup_store(struct device *dev,
 	if (kstrtoll(buf, 0, &interval))
 		return -EINVAL;
 
-	mutex_lock(&sysfs_lock);
+	guard(mutex)(&sysfs_lock);
 
 	if (fsl_wakeup->timer) {
 		disable_irq_wake(fsl_wakeup->timer->irq);
 		mpic_free_timer(fsl_wakeup->timer);
 		fsl_wakeup->timer = NULL;
 	}
 
-	if (!interval) {
-		mutex_unlock(&sysfs_lock);
+	if (!interval)
 		return count;
-	}
 
 	fsl_wakeup->timer = mpic_request_timer(fsl_mpic_timer_irq,
 						fsl_wakeup, interval);
-	if (!fsl_wakeup->timer) {
-		mutex_unlock(&sysfs_lock);
+	if (!fsl_wakeup->timer)
 		return -EINVAL;
-	}
 
 	ret = enable_irq_wake(fsl_wakeup->timer->irq);
 	if (ret) {
 		mpic_free_timer(fsl_wakeup->timer);
 		fsl_wakeup->timer = NULL;
-		mutex_unlock(&sysfs_lock);
-
 		return ret;
 	}
 
 	mpic_start_timer(fsl_wakeup->timer);
 
-	mutex_unlock(&sysfs_lock);
-
 	return count;
 }