iwlwifi: Take the fw file name from the iwl_cfg.

This patch adds fw_name to iwl_cfg. This allows run time selection
of needed fw/ucode file

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>