main: rotor: erase immediately after send

- improves trace diagnostic output by moving cursor back over the
the rotor before a diagnostic message has a chance to be printed.
there is still a race condition, but it is much better.

Change-Id: Iad7767f2a5dbbd67b0f33b9bfc2c3864ce308990

firmware/apps/cardem/main.c

@@ -194,8 +194,8 @@ extern int main(void)
  WDT_Restart(WDT);
 #if TRACE_LEVEL >= TRACE_LEVEL_DEBUG
  const char rotor[] = { '-', '\\', '|', '/' };
- putchar('\b');
  putchar(rotor[i++ % ARRAY_SIZE(rotor)]);
+ putchar('\b');
 #endif
  check_exec_dbg_cmd();
  osmo_timers_prepare();