siglent-sds: Fix SR_CONF_TRIGGER_SLOPE config_set() code.

siglent-sds: Fix SR_CONF_TRIGGER_SLOPE config_set() code.

Also use "r" and "f" as trigger slope values for now, since that's
what most other drivers do currently.

This also fixes two scan-build issues:

  api.c:559:3: warning: Value stored to 'tmp_str' is never read
                  tmp_str = g_variant_get_string(data, NULL);
                  ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  api.c:561:9: warning: Use of memory after it is freed
                  ret = siglent_sds_config_set(sdi, "%s:TRSL %s",
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~