* The function returns a 16-bit integer whose high-order byte contains status bits.
* The meaning of the low-order byte varies, depending on the cmd value.
* The high-order bits [ Line Status Register (LSR) ]are as follows:
* Bit Meaning if Set
* 15 Timed out Error in Received FIFO
* 14 Transmission-shift register empty Empty Data Holding Registers (RxRDY)
* 13 Transmission-hold register empty Empty Transmitter Holding Register (TxRDY)
* 12 Break detected Break Interrupt
* When service is _COM_INIT or _COM_STATUS, the low-order bits are defined as follows:
* Bit Meaning if Set
* 7 Receive-line signal detected
* 6 Ring indicator
* 5 Data-set-ready 0x20
* 4 Clear-to-send
* 3 Change in receive-line signal detected
* 2 Trailing-edge ring indicator
* 1 Change in data-set-ready status
* 0 Change in clear-to-send status