demo: Properly stop acquisition after reaching our sample quota

demo: Properly stop acquisition after reaching our sample quota

The demo driver was using sr_session_source_add_channel() to add
a poll source, but was relying on sr_session_run_poll() to call
sr_session_source_remove(). This, coupled with the design of the
driver caused errors once the samples were collected.

The error stream was most likely related to failing to properly close
one of the channels.

Signed-off-by: Alexandru Gagniuc <redacted>