iwlwifi: use iwl_cmd instead of iwl3945_cmd

This patch makes use of iwl_cmd instead of iwl3945_cmd and related
structures which were just the same.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>