wifi: iwlwifi: mvm: make flush code a bit clearer

The mask building here is only relevant for the old TX API,
so move it into the else branch.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230305124407.c0795543f254.I302124a8584dd049577b0c2c74ecd7c48ddf4f3e@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>