hameg-hmo: Send exactly one sigrok frame per scope frame

hameg-hmo: Send exactly one sigrok frame per scope frame

The previous implementation used to put FRAME_BEGIN and FRAME_END
markers around each received chunk of samples, while those chunks
correspond to a single channel (analog) or a group of eight channels
(digital) each. In other words, the hameg-hmo driver had provided a
multiple of the requested frames, and those frames were incomplete.

Make sure to only send FRAME_BEGIN before the first channel's data,
and FRAME_END after the last channel's data of a frame. Thus make
sigrok frames exactly match the scope's frames.

Add some comments on the frame marker and the acquisition stop logic
while we are here.