Originally, the 53c700 driver implemented queue_depth changing as an
attribute override, primarily as a demonstration of how it should be
done.  Now that a large number of drivers wish to implement this
functionality, it should become an API rather than an attribute
override, since the latter are supposed to be used as one off extensions
rather than the de-facto API.
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>