iwlwifi: use __get_str in tracing

Instead of using (char *)__get_dynamic_array use
__get_str. The latter is actually a macro that
expands to the former in the code, but trace-cmd
in userspace can parse __get_str only.

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