iwlwifi: fix situation in which debug message is printed

3945 does not have update_chain_flags defined and because if this we always
see the debug message that does not apply to it. Add a check to be specific
about what is actually happening.

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