cp#1000044 DOC import: fProtEnabled means document is not totally read-only

Reading the spec, it seems that we have two cases here:

- if lKeyProtDoc is set and fProtEnabled is 1, then editing forms is
  allowed
- if lKeyProtDoc is set, but fProtEnabled is 0, then the document is
  totally read-only

So in the first case avoid the SetModifyPasswordHash() call to allow
form field editing.

Change-Id: Id0c48e8eb4bde75c5520a7b108fcfada51148faf
(cherry picked from commit 8757bea2e88c6e349e1fe98d8e9695d7b9c6179e)
Reviewed-on: https://gerrit.libreoffice.org/8762
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>