sr_dev_clear(): Always free sdi->priv (devc).

sr_dev_clear(): Always free sdi->priv (devc).

Until now, clear_helper() callbacks for std_dev_clear_with_callback()
were expected to g_free(devc), but not all of them did that.

Have std_dev_clear_with_callback() unconditionally g_free(sdi->priv)
(i.e., devc), regardless of whether a clear_helper() callback was
provided or not. It was doing g_free(sdi->priv) when no callback
was provided already anyway.

This makes the individual drivers' clear_helper() implementations
shorter and prevents errors such as missing g_free(devc) calls.

This works, because all drivers either call std_dev_clear_with_callback()
directly, or indirectly via std_dev_clear().

This also allows us to remove some no-longer needed dev_clear()
and clear_helper() implementations that only did g_free(devc)
in favor of std_dev_clear().