Apply #pragma once, misc cleanup (#12322)

* Apply #pragma once in headers
* Adjust some thermistors formatting
* Misc cleanup and formatting

Marlin/src/HAL/HAL_AVR/fastio_AT90USB.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * Pin mapping (Teensy) for AT90USB646, 647, 1286, and 1287
@@ -28,9 +29,6 @@
  *            The logical pins 46 and 47 are not supported by Teensyduino, but are supported below as E2 and E3
  */
 
-#ifndef _FASTIO_AT90USB_H_
-#define _FASTIO_AT90USB_H_
-
 #include "fastio_AVR.h"
 
 // change for your board
@@ -697,5 +695,3 @@
 #define TIMER3A 5
 #define TIMER3B 4
 #define TIMER3C 3
-
-#endif // _FASTIO_AT90USB_H_