efidev_parent() currently assumes that any device with BUS_TYPE_EFI is
part of a struct efi_device.  This assumption is not valid, since the
code in efi_device_info() may also create a device with BUS_TYPE_EFI.

Fix by searching through the list of registered EFI devices when
looking for a match, instead of relying on the bus type value.

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