Bluetooth: CSIP: Update CSIP duplicate rank search

In the CSIP Set Coordinator verify_members we verify that
all members have unique ranks.

This was done as part of the initial loop, but it is
more efficient to do this as a separate loop as we can reduce
the number of items to compare, as well as guarding this properly
by !zero_rank.

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