iwlwifi: add struct iwl_cfg

This patch introduces struct iwl_cfg. struct iwl_cfg defines static
configuration for each device type and sku. It is passed as driver_data
to the bus probe function.
This patch also introduce new common header file iwl-core.h
which will represent core functionality of iwlwifi driver

3945ABG uses separate iwl-3945-dev.h header file for now

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>