Bluetooth: CSIP: Add missing and fix Set Coordinator set info

The set info returned by the Set Coordinator did not contain
information about the lockable state. Furthermore the
set info was not properly initiated, as it was missing the
rank, which was set somewhere else.

This commit adds the lockable state to the set info.
This commit removes the rank from internal structure,
and uses a reference to the set info instead, and only
store the rank there.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>