fix: correctly deserialize MavFrame

This commit addresses two bugs present in the `MavFrame::deser`
implementation. It parsed the sequence number after system and component
ids, but should instead parse it before. Furthermore the implementation
used the `get_u32_le` method to parse the message id in mavlink v2.
However the MAVLink spec specifies that this field should have 3 bytes,
not 4 (like `u32`).