[efi] Do not raise TPL within EFI_DRIVER_BINDING_PROTOCOL.Supported()

When booting some versions of the UEFI shell, our driver binding
protocol's Supported() entry point is called at TPL_NOTIFY for no
discernible reason.  Attempting to raise to TPL_CALLBACK triggers an
immediate assertion failure in the firmware.

Since our Supported() method can run at any TPL, fix by simply not
attempting to raise the TPL within this method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_driver.c

 static EFI_STATUS EFIAPI
 efi_driver_supported ( EFI_DRIVER_BINDING_PROTOCOL *driver __unused,
 		       EFI_HANDLE device, EFI_DEVICE_PATH_PROTOCOL *child ) {
-	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 	struct efi_driver *efidrv;
-	EFI_TPL saved_tpl;
 	int rc;
 
 	DBGCP ( device, "EFIDRV %s DRIVER_SUPPORTED",
 		return EFI_ALREADY_STARTED;
 	}
 
-	/* Raise TPL */
-	saved_tpl = bs->RaiseTPL ( TPL_CALLBACK );
-
 	/* Look for a driver claiming to support this device */
 	for_each_table_entry ( efidrv, EFI_DRIVERS ) {
 		if ( ( rc = efidrv->supported ( device ) ) == 0 ) {
 			DBGC ( device, "EFIDRV %s has driver \"%s\"\n",
 			       efi_handle_name ( device ), efidrv->name );
-			bs->RestoreTPL ( saved_tpl );
 			return 0;
 		}
 	}
 	DBGCP ( device, "EFIDRV %s has no driver\n",
 		efi_handle_name ( device ) );
 
-	bs->RestoreTPL ( saved_tpl );
 	return EFI_UNSUPPORTED;
 }