ieee1394: conditionally export ieee1394_bus_type

Follow-up to patch "Consolidate driver registering":
Since I plan the lifetime of Linux 2.6.20 to be the deprecation phase
of CONFIG_IEEE1394_EXPORT_FULL_API, it seems fair to keep all previously
exported symbols available with this option until this phase is over.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>