iwlwifi/iwl3945: unify rts_tx_cmd_flag

3945 and 4965 share the functionality for setting RTS and CTS to
the tx_cmd. Unify these functions and move the common
functionality to core.

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>