iwlagn: simplify the bus architecture

Call iwl_probe with a ready iwl_bus struct. This means that the bus layer
assigns the irq, dev and iwl_bus_ops pointers to iwl_bus before giving it to
iwl_probe.

The device specific struct is allocated together with the common iwl_bus struct
by the bus specific layer. The pointer to the aggregate struct is passed to the
upper layer that holds a pointer to iwl_bus instead of an embedded iw_bus.
The private data given to the PCI subsystem is now iwl_bus and not iwl_priv.

Provide bus_* inliners on the way in order  to simplify the syntax.

Rename iwl-pci.h -> iwl-bus.h since it is bus agnostic and represent the
external of the bus layer.

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