iwlwifi: mvm: cleanups in iwl_dbgfs_frame_stats_read

Switch pos to char * which makes the code a bit shorter
as well as other minor cleanups suggested by Joe Perches.

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