iwlwifi: mvm: Don't sleep in RX path

Don't use cancel_delayed_work_sync() inside the channel switch
notifications as they are handled synchronously as part of the RX path.
Fix that by replacing it with cancel_delayed_work(). This should be safe
as we don't really care whether the work is already started and in such
case we would disconnect anyway.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>