settings: add a comment to remind about QUBIK_FLASH_MAGIC_VAL
QUBIK_FLASH_MAGIC_VAL is a random unique value, that has to be modified every time struct qubik_settings is changed. Add a comment to remind about that.
QUBIK_FLASH_MAGIC_VAL is a random unique value, that has to be modified every time struct qubik_settings is changed. Add a comment to remind about that.