Clean up fast_pwm.cpp

Marlin/src/HAL/HAL_AVR/fast_pwm.cpp

-#ifdef __AVR__
-
-#include "../../inc/MarlinConfigPre.h"
 /**
- * get_pwm_timer
- *  Grabs timer information and registers of the provided pin
- *  returns Timer struct containing this information
- *  Used by set_pwm_frequency, set_pwm_duty
+ * Marlin 3D Printer Firmware
+ * Copyright (C) 2019 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#ifdef __AVR__
+
+#include "../../inc/MarlinConfigPre.h"
 
 #if ENABLED(FAST_PWM_FAN)
+
 #include "HAL.h"
 
 struct Timer {
@@ -20,7 +35,13 @@
   uint8_t q;                    // the timer output [0->2] (A->C)
 };
 
-  Timer get_pwm_timer(pin_t pin) {
+/**
+ * get_pwm_timer
+ *  Get the timer information and register of the provided pin.
+ *  Return a Timer struct containing this information.
+ *  Used by set_pwm_frequency, set_pwm_duty
+ */
+Timer get_pwm_timer(const pin_t pin) {
   uint8_t q = 0;
   switch (digitalPinToTimer(pin)) {
     // Protect reserved timers (TIMER0 & TIMER1)