Set TIMER_SERIAL to TIM9

Default TIMER_SERIAL assignment is to TIM7, which conflicts with TIMER_SERVO.

Author: chrissbarr

variants/RUMBA32_F446VE/variant.h

@@ -135,6 +135,7 @@ extern "C" {
 // Use TIM6/TIM7 when possible as servo and tone don't need GPIO output pin
 #define TIMER_TONE              TIM6
 #define TIMER_SERVO             TIM7
+#define TIMER_SERIAL            TIM9
 
 // UART Definitions
 #define SERIAL_UART_INSTANCE    1 // Connected to EXP3 header