iwlwifi: iwl-agn-rs: initialize rs with valid antenna

This patch fix rate scaling initialization. Rate scaling
was initialized always with B antenna.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>