Change assert to not fire when intentionally disabling PCI enumeration (

#572)

## Description

When setting PcdPciDisableBusEnumeration to TRUE, an assert is hit in
StartPciDevices due to the RootBridge not existing. This is expected
when skipping enumeration so change the assert to only fail if
enumeration was not disabled.

- [ ] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?

## How This Was Tested

Tested on QEMU Q35 platform

## Integration Instructions

N/A

MdeModulePkg/Bus/Pci/PciBusDxe/PciDeviceSupport.c

@@ -888,7 +888,8 @@ StartPciDevices (
 
   RootBridge = GetRootBridgeByHandle (Controller);
   if (RootBridge == NULL) {
-    ASSERT (RootBridge != NULL);
+    // ASSERT (RootBridge != NULL);                    // MU_CHANGE
+    ASSERT (PcdGetBool (PcdPciDisableBusEnumeration)); // MU_CHANGE
     return EFI_NOT_READY;
   }