Fix resetting of access level when updating workflow

This is the same fix as 63bbb19393c3a2b9bdc7e159992c160561a7ce09, and
addresses the incorrect behavior for the fields

• Who can change workflow
• Who can change access levels
  by applying the same mechanism used to detect a change in the status to
  the access level, i.e. comparing the updated access level with the
  parent and not the current value.

Fixes 0016771

Signed-off-by: Damien Regad dregad@mantisbt.org

Original commit message reworded