Apply #pragma once, misc cleanup (#12322)

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

Marlin/src/feature/fanmux.h

@@ -19,15 +19,11 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * feature/fanmux.h - Cooling Fan Multiplexer support functions
  */
 
-#ifndef _FANMUX_H_
-#define _FANMUX_H_
-
 extern void fanmux_switch(const uint8_t e);
 extern void fanmux_init(void);
-
-#endif // _FANMUX_H_