The attached patch illustrates how the new attribute functions can be
used.

The 53c700 modifies the queue_depth attribute so it can be set by the
root user, and also adds a new active_tags attribute which may be used
to obtain the information it currently puts out via its proc interface.

This achieves all I think we really need, which is the ability to have
LLDs provide their own routines for setting the default attributes.