BUILD: task: Fix build when no 8B CAS is available at all

BUILD: task: Fix build when no 8B CAS is available at all

clang (rightfully) complains that when neither HA_CAS_IS_8B nor
HA_HAVE_CAS_DW is defined, there is a mismatch when we're doing a CAS,
as we're mixing unsigned int and int.
To fix that, just turn old_state to an unsigned int, as it should be.

This should fix github issue #3441