iwlwifi: mvm: do string formatting in debug triggers

The current code has a lot of duplicates of printing into a buffer
(while having to make sure it's NUL-filled and -terminated) and
then passing that to the debug trigger collection.

Since that's error-prone, instead make the debug trigger collection
function take a format string and format arguments (with compiler
validity checking) and handle the buffer internally.

This makes one behavioural change -- instead of sending the whole
buffer to userspace (clearing is needed to not leak stack data) it
just passes the actual string (including NUL-terminator.)

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