iwlwifi: add iwl-core module

This patch adds iwl-core module to iwlwifi driver.
This module will contain common code and infrastracture for iwlwifi
driver

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>