Improve RSCP frame reception handling to ensure complete frames are processed

[torbennehmer]

Tag EH_REQ_GET_SAVED_ERRORS returns a large amount of data (approx 4k on my end), which gets split by TCP in multiple chunks.

Sample code:

print("query EH_REQ_GET_SAVED_ERRORS")
result = e3dc.sendRequest((RscpTag.EH_REQ_GET_SAVED_ERRORS, RscpType.NoneType, None))
print(json.dumps(result, indent=2, default=str))

This yields:

Traceback (most recent call last):
  File "/home/torben/src/python-e3dc/e3dc/_e3dc_rscp_local.py", line 97, in sendRequest
    receive = self._receive()
  File "/home/torben/src/python-e3dc/e3dc/_e3dc_rscp_local.py", line 75, in _receive
    decData = rscpDecode(self.encdec.decrypt(data))[0]
              ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/torben/src/python-e3dc/e3dc/_rscpLib.py", line 262, in rscpDecode
    return rscpDecode(rscpFrameDecode(data)[0])
                      ~~~~~~~~~~~~~~~^^^^^^
  File "/home/torben/src/python-e3dc/e3dc/_rscpLib.py", line 223, in rscpFrameDecode
    data, crc = struct.unpack(
                ~~~~~~~~~~~~~^
        "<" + str(length) + "s" + crcFmt,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        frameData[struct.calcsize(headerFmt) : totalLen],
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
struct.error: unpack requires a buffer of 4216 bytes

Altering the receive call to a loop fixes the problem, it assembles the frame appropriately. Tested various other calls to ensure correct functionality.

Prerequisite for hacs-e3dc PR torbennehmer/hacs-e3dc#373

[torbennehmer]

#146 looks like the more stable solution to me. closing