link-mso19: Use default dev_clear() handler

link-mso19: Use default dev_clear() handler

The link-mso19 open-codes the std_dev_clear() function and uses it as its
custom dev_clear() callback. The std_dev_clear() function is automatically
called if no custom dev_clear() callback is specified, so just drop custom
implementation and use the default.

This also fixes a memory leak where the link-mso19 driver did not properly
free its driver state struct in the dev_clear() callback.

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