Fix STM32 + SoftwareSerial compile (#17831)

Marlin/src/HAL/STM32/timers.cpp

@@ -31,9 +31,6 @@
 
 #define NUM_HARDWARE_TIMERS 2
 
-#ifndef SWSERIAL_TIMER_IRQ_PRIO
-  #define SWSERIAL_TIMER_IRQ_PRIO 1
-#endif
 #ifndef STEP_TIMER_IRQ_PRIO
   #define STEP_TIMER_IRQ_PRIO 2
 #endif
@@ -41,6 +38,13 @@
   #define TEMP_TIMER_IRQ_PRIO 14   // 14 = after hardware ISRs
 #endif
 
+#if HAS_TMC_SW_SERIAL
+  #include <SoftwareSerial.h>
+  #ifndef SWSERIAL_TIMER_IRQ_PRIO
+    #define SWSERIAL_TIMER_IRQ_PRIO 1
+  #endif
+#endif
+
 #ifdef STM32F0xx
   #define HAL_TIMER_RATE (F_CPU)      // Frequency of timer peripherals
   #define MCU_STEP_TIMER 16
@@ -175,4 +179,8 @@ TIM_TypeDef * HAL_timer_device(const uint8_t timer_num) {
   return nullptr;
 }
 
+void SetSoftwareSerialTimerInterruptPriority() {
+  TERN_(HAS_TMC_SW_SERIAL, SoftwareSerial::setInterruptPriority(SWSERIAL_TIMER_IRQ_PRIO, 0));
+}
+
 #endif // ARDUINO_ARCH_STM32 && !STM32GENERIC