iwlwifi: mvm: use switching_chanctx argument instead of csa_active

Now that all CSA flows are using the switch_vif_chanctx op, we can
rely on the switching_chanctx boolean that is passed to the
__iwl_mvm_assign_vif_chanctx() and __iwl_mvm_unassign_vif_chanctx()
functions to decide whether the context switch flows need to be
executed.  In this way we make the chanctx switch flow more generic,
without having to rely on the csa_active flag being set.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>