iwlwifi: use configured valid rx chain for scan

Use configured valid rx chains in scan command instead of ANT_ABC, correcting
valid rx chain configuration of 4965, should be ANT_ABC instead of ANT_AB.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>