Bluetooth: CSIP: Set Coordinator move data to instances

Moved the busy flag and the GATT paramters to the
individual CSIP Set Coordinator instances.

This allows to use multiple instances in parallel,
and thus in turn allow CAP to do things in parallel.

This commit has been done with minimal amount of changes,
even if there are multiple pieces of code that could likely
be refactored to be better.

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