Bluetooth: CSIP: Avoid multiple conn CBs registers

The bt_csip_set_member_register function would register
the connection, and connection auth info, callbacks for
each CSIS instance, which could give errors when/if the
callbacks are called too many times. In any case, they
should not be registered multiple times.

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