Bug 37165: Can't edit frequencies due to stray cud- in modify op

Bug 37165: Can't edit frequencies due to stray cud- in modify op

The op modify in subscription-frequencies.pl isn't a cud- operation, it's
a GET of the editing form, so it shouldn't have been changed to cud-modify.

Test plan:
1. Don't apply the patch yet
2. Serials - Manage frequencies - for any frequency click Edit
2. Note that the editing form is empty
3. Apply patch, restart_all
4. Repeat step 2, note that now the editing form has the existing data
5. Change anything, Display order is nice, and save to verify that
   cud-savemod still works

Signed-off-by: Brendan Lawlor <blawlor@clamsnet.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>