serial-dmm: Properly handle continuous mode

serial-dmm: Properly handle continuous mode

serial-dmm does not check if a sample limit is actually in place before deciding
to stop acquisition. Since the sample limit is set at 0 by default, operating
in continuous mode will cause acquisition to stop before even sending the first
sample.

Check to make sure we actually are in a sample-limited mode before stopping for
this reason.

Signed-off-by: Alexandru Gagniuc <redacted>