Bluetooth: CSIP: Shell: Fix discover_members

The cmd_csip_set_coordinator_discover_members did not properly
handle the members_found and addr_found values.

It has been modified to run through all known values
before scanning, and set the value appropriately.

This also fixes a minor bug where err = 0 was missing.

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