Kernel: Fix UHCIController singleton startup null-deref race condition.

The following KUBSAN crash on startup was reported on discord:

```
UHCI: Started
KUBSAN: reference binding to null pointer of type struct UHCIController
KUBSAN: at ../../Kernel/Devices/USB/UHCIController.cpp, line 67
```

After inspecting the code, it became clear that there's a window of time
where the kernel task which monitors the UHCI port can startup and start
executing before the UHCIController constructor completes. This leaves
the singleton pointing to nullptr, thus in the duration of this race
window the "UHCI port proc" thread will go an and de-reference the null
pointer when trying to read for status changes on the UHCI root ports.

Reported-by: @stelar7
Reported-by: @bcoles

Fixes: SerenityOS#6154

Kernel/Devices/USB/UHCIController.cpp

@@ -77,8 +77,10 @@ UNMAP_AFTER_INIT void UHCIController::detect()
  return;
 
  if (PCI::get_class(address) == 0xc && PCI::get_subclass(address) == 0x03 && PCI::get_programming_interface(address) == 0) {
- if (!s_the)
+ if (!s_the) {
  s_the = new UHCIController(address, id);
+ s_the->spawn_port_proc();
+ }
  }
  });
 }
@@ -93,8 +95,6 @@ UNMAP_AFTER_INIT UHCIController::UHCIController(PCI::Address address, PCI::ID id
 
  reset();
  start();
-
- spawn_port_proc();
 }
 
 UNMAP_AFTER_INIT UHCIController::~UHCIController()
@@ -134,7 +134,7 @@ void UHCIController::reset()
 
 UNMAP_AFTER_INIT void UHCIController::create_structures()
 {
- // Let's allocate memory for botht the QH and TD pools
+ // Let's allocate memory for both the QH and TD pools
  // First the QH pool and all of the Interrupt QH's
  auto qh_pool_vmobject = ContiguousVMObject::create_with_size(2 * PAGE_SIZE);
  m_qh_pool = MemoryManager::the().allocate_kernel_region_with_vmobject(*qh_pool_vmobject, 2 * PAGE_SIZE, "UHCI Queue Head Pool", Region::Access::Write);