[chirp_devel] [uv-b5] fixing ack issues permanently

Taking a stab at fixing the "ack-a-mole" issues. Rather than attempting to maintain a list of known acks, switching to a solution to capture first ack received, and use that through rest of download session. This should still fail if, for some reason, ack changes within a download session as this would likely indicate a real problem (communication errors, etc).

Would like to get feedback from a few uv-b5/b6 users to verify this is working ok before I submit a patch.

-Jens AF5MI