Followup to #12249

Marlin/src/sd/cardreader.h

     #if NUM_SERIAL > 1
       uint8_t transfer_port;
     #else
-      constexpr uint8_t transfer_port = 0;
+      static constexpr uint8_t transfer_port = 0;
     #endif
   #endif
 

buildroot/share/tests/megaatmega2560_tests

 opt_set POWER_SUPPLY 1
 opt_set GRID_MAX_POINTS_X 16
 opt_enable PIDTEMPBED FIX_MOUNTED_PROBE Z_SAFE_HOMING \
-           REPRAP_DISCOUNT_SMART_CONTROLLER SDSUPPORT LCD_PROGRESS_BAR LCD_PROGRESS_BAR_TEST EEPROM_SETTINGS PINS_DEBUGGING \
+           SDSUPPORT EEPROM_SETTINGS FAST_FILE_TRANSFER \
-           BLINKM PCA9632 RGB_LED NEOPIXEL_LED AUTO_POWER_CONTROL \
+           REPRAP_DISCOUNT_SMART_CONTROLLER LCD_PROGRESS_BAR LCD_PROGRESS_BAR_TEST \
+           BLINKM PCA9632 RGB_LED NEOPIXEL_LED AUTO_POWER_CONTROL PINS_DEBUGGING \
            NOZZLE_PARK_FEATURE FILAMENT_RUNOUT_SENSOR FILAMENT_RUNOUT_DISTANCE_MM \
            AUTO_BED_LEVELING_LINEAR Z_MIN_PROBE_REPEATABILITY_TEST DEBUG_LEVELING_FEATURE \
            SKEW_CORRECTION SKEW_CORRECTION_FOR_Z SKEW_CORRECTION_GCODE