Bluetooth: CSIP: Set member: Fix issue with re-registration

The bt_csip_set_member_register kept a counter that was not
decreased when bt_csip_set_member_unregister was called.
This meant that we could register and unregister CSIS,
but we could not re-register once it had been unregistered.

This commit fixes this by removing the counter and instead
rely on the service instance state, which also requires restoring
the original service definition, as well as adding a test that would
have failed with the previous version.

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