Consistency around SfxUInt16Item in svx/svx3ditems.hxx

...similar to what has been done for svx/sdtmfitm.hxx in
68969cc61adecac481ae9656978ef952f435b310 "Consistency around SdrMetricItem."

Change-Id: I2e467df03a34d694214e0560476cb7fd76ef0735