std_serial_dev_acquisition_stop(): Remove serial parameter

All callers of std_serial_dev_acquisition_stop() currently pass sdi->conn
for the serial parameter. And the other std_serial helper functions already
require that the conn field of the sr_driver_inst passed to the functions
points to the sr_serial_dev_inst associated with the device.

Modify std_serial_dev_acquisition_stop() to follow the same pattern and
remove the serial parameter. This simplifies the API and ensures consistent
behaviour between different drivers.

Signed-off-by: Lars-Peter Clausen <redacted>