iwlwifi: work around bogus active chains detection

The current algorithm will sometimes "detect" that
more chains are enabled than are really present in
the device because, for unknown reasons, the ucode
sends up all-zeroes signal values.

The simplest way of solving this is to restrict the
active chains mask to the chains we know are really
present on the device.

This fixes a bug with some devices where, since sometimes
more chains are enabled than really present, the system would hang.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>