SKR E3 Mini V2.0 Fan Bug Fix (#21079)

Co-authored-by: Scott Lahteine <github@thinkyhead.com>

Marlin/src/pins/stm32f1/pins_BTT_SKR_MINI_E3_V2_0.h

 
 #define SKR_MINI_E3_V2
 
+#define BOARD_CUSTOM_BUILD_FLAGS -DTONE_CHANNEL=4 -DTONE_TIMER=4
+
 // Onboard I2C EEPROM
 #if NO_EEPROM_SELECTED
   #define I2C_EEPROM