iwlwiif: mvm: refactor iwl_mvm_notify_rx_queue

Instead of allocating memory for which we have an upper
limit, use a small buffer on stack.

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