iwlagn: introduce iwl_bus and iwl_bus_ops

iwl_bus will represent a bus, and iwl_bus_ops all the operations that can be
done on this bus.
For the moment only set_prv_data is implemented. More to come...

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>