Fixup chunk offset movement on resend

Since we are moving the offset backwards on failures, it doesnt
matter whether we have seen a failure or success when incrementing
offset. We always need to move it to indicate a chunk is in flight,
otherwise canSendNextChunk() will allow us to send the same chunk
again and again even when it is already inflight.

Change-Id: If5dfd1913bda4328c5dfab0869f2df697bbd3cfc
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>