efi_loader: disk: not delete BLK device for BLK(IF_TYPE_EFI_LOADER) devices

When we create an efi_disk device with an UEFI application using driver
binding protocol, the 'efi_driver' framework tries to create
a corresponding block device(UCLASS_BLK/IF_TYPE_EFI). This will lead to
calling a PROBE callback, efi_disk_probe().
In this case, however, we don't need to create another "efi_disk" device
as we already have this device instance.

So we should avoid recursively invoke further processing in the callback
function.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>