Fix #37106: Hair Length vanishes when Advanced is enabled

Seems to be just a missing case from r34687.

Could not see a reason why hair length is to be hidden in
advanced settings mode.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60817 954f8c5b-7b00-dc11-b283-0030488c597c