iwl3945: Use iwl-hcmd host command routines

With the previously added tfd related ops, we can now use the iwl-tx.c host
command enqueue routine. Since the 3945 host command specific
routines are identical to the agn ones, we can just remove them from the 3945
code.

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>