Stm32f4UsbSerial: Report RX overrun when the RX buffer is full.
Stm32f4UsbSerial: Report RX overrun when the RX buffer is full.

Even though transfer is reliable we must still report overrun when the receive buffer is full so that the SerialModule knows when a command is too long and does not lock up processing. This is already done like this in other USB-based serial drivers.

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