Reduce STM32F4 compile warnings

Marlin/src/HAL/HAL_STM32F4/HAL.h

   #define NUM_SERIAL 1
 #endif
 
+#undef _BV
 #define _BV(b) (1 << (b))
 
 /**

Marlin/src/HAL/HAL_STM32F4/fastio_STM32F4.h

 #ifndef _FASTIO_STM32F4_H
 #define _FASTIO_STM32F4_H
 
+#undef _BV
 #define _BV(b) (1 << (b))
 
 #define USEABLE_HARDWARE_PWM(p) true

Marlin/src/core/macros.h

 #define NANOSECONDS_PER_CYCLE (1000000000.0 / F_CPU)
 
 // Remove compiler warning on an unused variable
-#define UNUSED(x) (void) (x)
+#define UNUSED(x) ((void)(x))
 
 // Macros to make a string from a macro
 #define STRINGIFY_(M) #M