Fix QSpinBox clipping issue on Mac when frame=false

This was a regression from 5.0. Since we still require a minimum
height to be present in order for the spin buttons to draw correctly,
we require the same height from a spinbox without a frame as one
with a frame.

task-number: QTBUG-31538
Change-Id: I999f59a29e826e8e621faebffce9cccd9bedb8ca
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>