button_handler: fix invalid power event error

Current power button handler handles longPowerReleased event, which
should be longPowerPressed.
revert back to longPowerPressed to avoid error of invalid power event.

Tested on Bletchley:

- Before
```
root@bletchley:~# journalctl -u phosphor-button-handler
Dec 19 21:52:53 bletchley button-handler[521]: Multi-host system detected : 6
Dec 19 21:52:53 bletchley button-handler[521]: 1 is invalid power event. skipping...
```

- After
```
root@bletchley:~# journalctl -u phosphor-button-handler
Dec 19 21:34:58 bletchley button-handler[418]: Multi-host system detected : 6
Dec 19 21:34:58 bletchley button-handler[418]: handlePowerEvent : handle long power button press
```

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Change-Id: I77977f23f9e669cd4a8785608f1cb3ca4d148bed
diff --git a/src/button_handler.cpp b/src/button_handler.cpp
index 29ae1bc..38649f0 100644
--- a/src/button_handler.cpp
+++ b/src/button_handler.cpp
@@ -207,7 +207,7 @@
 
     // ignore  power and reset button events if BMC is selected.
     if (isMultiHostSystem && (hostNumber == BMC_POSITION) &&
-        (powerEventType != PowerEvent::longPowerReleased))
+        (powerEventType != PowerEvent::longPowerPressed))
     {
         lg2::info(
             "handlePowerEvent : BMC selected on multi-host system. ignoring power and reset button events...");
@@ -232,7 +232,7 @@
 
             break;
         }
-        case PowerEvent::longPowerReleased:
+        case PowerEvent::longPowerPressed:
         {
             dbusIfaceName = chassisIface;
             transitionName = "RequestedPowerTransition";