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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>