iwl3945: Use iwl-agn-rs.h rates definitions.

A lot of rate relates definition are shared between iwl-3945-rs.h and
iwl-agn-rs.h. Let's just use the agn version, and add the 3945 specific
constants there.

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