wifi: iwlwifi: pcie: point invalid TFDs to invalid data

There are occasionally bugs which cause the device to try
to use a TFD that it wasn't supposed to, and these are
very hard to diagnose. Fill all unused TFDs with a debug
command that immediately causes an error to be detected
in these cases.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230816104355.10a9af1ca91f.Ifc790d62c52b4bc9a74c9581610af498509f5759@changeid


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