iwlagn: add get_dev to iwl_bus_ops

Bus specific layer must know how to return the struct device* of the device.
Implement that as a callback of iwl_bus_ops and use that callback instead of
using the priv->pdev pointer which is meant to disappear soon.

Since the struct device * is needed in hot path, iwl_bus holds a pointer to it
instead of calling get_dev all the time.

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