Bluetooth: Host: Missing unref for PAST sync_info

When we supply the conn pointer of the remote device that
provides the PAST information, we do a lookup on the
conn_handle using bt_conn_lookup_handle which takes a
reference count. This count is never dereferenced again.

This commit adds calls to bt_conn_unref to ensure that we
do not keep the reference count after the function
returns.

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