disks: allow get_disks without smart checks

disks: allow get_disks without smart checks

the smart checks are only needed for the API call(s) that
list all disks and their status, but get_disks is also used
in disk usage checks and in the Ceph code, where the smart
status is completely irrelevant.

drop the implicit skipping of smart checks if $disk is set,
since we have an explicit parameter for this now.