Sending an SR_DF_META packet at the start of every stream is not
mandatory; the frontend should ask for what it needs, and any extra
information the driver wants to send will come in due time.
Currently hw_info_get() can receive requests for entries (info_id) that
the specific driver doesn't support. That is (right now) a valid
use-case and not an error (might change later, though).
Thus, for now, don't output messages for such requests at all (certainly
not as sr_err() where they show up in e.g. sigrok-cli output per default).
Check whether a sample limit was actually set (> 0) before checking if
that sample limit is reached. This also fixes continuous acquisition mode
for drivers which have that.