iwlwifi: mvm: small debugfs cleanups

Just clean up the code a bit, in particular
 * make all the debugfs writes follow the same pattern
   with respect to buf/buf_size variables
 * get rid of useless comments
 * check return values of all file creations
 * drop unnecessary parentheses
 * remove an unused struct definition
 * fix some whitespace

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