iwlwifi: mvm: save low latency causes in an enum

Currently we have a boolean variable for each cause.

This costs space, and requires to check each separately
when determining low latency.

Since we have another cause incoming, convert it to an enum.

While at it, move the retrieval of the prev value and the
assignment of the new value to be inside iwl_mvm_update_low_latency
and save the need for each caller to do it separately.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>