iwlwifi: mvm: remove superfluous flush_work()

Since iwl_mvm_start_p2p_roc() is only called from iwl_mvm_roc(),
which already flushes the same work item, doing it again in it
is superfluous. Remove it and move the comment to the first one.

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