iwlwifi: Add TFD library operations

The TFD structures for 3945 and agn HWs are fundamentally different. We thus
need to define operations for attaching and freeing them. This will allow us
to share a fair amount of code (cmd and tx queue related) between both
drivers.

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