Fix Mightyboard PIO env (#18098)

platformio.ini

 lib_deps          = ${common.lib_deps}
   TMC26XStepper=https://github.com/trinamic/TMC26XStepper/archive/master.zip
 src_filter        = ${common.default_src_filter} +<src/HAL/AVR>
+upload_speed      = 57600
 
 #
 # MightyBoard ATmega2560 (MegaCore 100 pin boards variants)
 lib_deps          = ${common.lib_deps}
   TMC26XStepper=https://github.com/trinamic/TMC26XStepper/archive/master.zip
 src_filter        = ${common.default_src_filter} +<src/HAL/AVR>
+upload_protocol   = wiring
+upload_speed      = 57600
+board_upload.maximum_size = 253952
 
 #
 # RAMBo