iwlagn: reduce off channel reception for 4965

Force use of chains B and C (0x6) for Rx for 4965
Avoid A (0x1) because of its off-channel reception on A-band.

Signed-off-by: Rick Farrington <rickdic@hotmail.com>
Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>