ci: fix IAR Pe111 and stm32h7s3 flash overflow

- examples/device/midi2_device/src/main.c: drop the unreachable
  `return 0;` after the `while(1)` superloop. IAR with
  --warnings_are_errors rejects Pe111 (statement is unreachable);
  C99 lets `int main` fall off the end, matching midi_test.

- examples/host/msc_file_explorer_freertos/skip.txt: skip
  stm32h7s3nucleo. The board has only 64 KB on-chip FLASH and the
  FreeRTOS + FatFS host MSC explorer now overflows by ~248 bytes
  after the async control queue refactor.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/device/midi2_device/src/main.c

@@ -715,6 +715,4 @@ int main(void) {
       }
     }
   }
-
-  return 0;
 }

examples/host/msc_file_explorer_freertos/skip.txt

@@ -1,3 +1,4 @@
 mcu:CH32F20X
 board:lpcxpresso54114
 mcu:FT90X
+board:stm32h7s3nucleo