iwl3945: Define send_tx_power

We can define this iwl_lib ops for 3945, as this would help us cleaning the
scanning 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>