M17 packet mode needs an update

[sp5wwp]

We have recently updated the M17 protocol's specification document. SDRangel has issues with decoding packets. Could you apply a few minor adjustments to the code to accommodate for that?

The SMS should have the following structure (packet data contents): 0x05 (text message designator), text message (UTF-8 encoded), 0x00 (null-termination), 2-byte CRC (same as for the rest of the protocol).

The message is segmented into 25-byte chunks. If there's not enough data left, the rest is zero-padded. We kept the bit order for the Packet Metadata Field you used before - EOT bit is 1<<7 and the data byte counter goes to <<2. I believe the CRC has wrong endianness though. Most significant byte should be at position [0] of the byte array.