iwlwifi: mvm: track low-latency sources separately

To be able to test low-latency behaviour properly, split the
different low-latency sources so that setting any one of them,
for example from debugfs, is sufficient; this avoids getting
the debug setting overwritten by other sources.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>