iwlwifi: clear iwl_cmd_meta structure before use

Resolve an issue in which out-dated fields in iwl_cmd_meta
could be used for later hardware commands.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>