Fix checksum field

[michael-myers]

Fixes #3 by ensuring the same checksum calculation in Encode() that is done in Checksum(). I think the code was meant to skip 2 bytes of the raw data, not the content data. It should start on byte 0 of the content data.

Regarding the output I was getting in the issue description. The output, with this fix:

2023/12/31 02:08:39 Starting Device
2023/12/31 02:08:39 Opening USB device
2023/12/31 02:08:39 USB Device opened
2023/12/31 02:08:39 Loop Started
2023/12/31 02:08:39 Writing:  [0xA4, 0x01, 0x4A, 0x00, 0xEF]
2023/12/31 02:08:39 Read: AntMessage (0x6F) [0x20]
2023/12/31 02:08:40 Writing:  [0xA4, 0x09, 0x46, 0x00, 0xB9, 0xA5, 0x21, 0xFB, 0xBD, 0x72, 0xC3, 0x45, 0x64]
2023/12/31 02:08:40 Writing:  [0xA4, 0x03, 0x42, 0x00, 0x00, 0x01, 0xE4]
2023/12/31 02:08:40 Writing:  [0xA4, 0x05, 0x51, 0x00, 0xC2, 0x00, 0x77, 0x00, 0x45]
2023/12/31 02:08:40 Read: AntMessage (0x40) [0x00, 0x46, 0x00]
2023/12/31 02:08:40 Writing:  [0xA4, 0x02, 0x45, 0x00, 0x39, 0xDA]
2023/12/31 02:08:40 Read: AntMessage (0x40) [0x00, 0x42, 0x00]
2023/12/31 02:08:40 Writing:  [0xA4, 0x03, 0x43, 0x00, 0x00, 0x20, 0xC4]
2023/12/31 02:08:40 Writing:  [0xA4, 0x02, 0x5B, 0x00, 0x01, 0xFC]
2023/12/31 02:08:40 Read: AntMessage (0x40) [0x00, 0x51, 0x00]
2023/12/31 02:08:40 Loop stopped!
2023/12/31 02:08:40 Closing USB device
2023/12/31 02:08:40 USB Device closed

[purpl3F0x]

Does this work if you remove line 86 checksum := MESG_TX_SYNC ^ byte(msgLen) ^ m.Id0 ?? Cause if I don't confuse something all bits are xored correctly

[purpl3F0x]

Actually I think it shoull be n:=3 try this one, my ant dongle seems it doesnt care at all :P