iwlwifi: mvm: let any command flag be passed to iwl_mvm_flushtx_path()

Instead of only allowing the caller to decide whether the CMD_ASYNC
flag is set, let it pass the entire flags bitmask.  This allows more
flexibility and will be needed when we call this function in the
suspend flow (where other flags are needed).

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