[psm] Fix calculation for clickpad softbuttons at the top

On laptops like the ThinkPad X240, ClickPad buttons are located at the
top. The hw.psm.synaptics.softbuttons_y sysctl was supposed to allow this
by setting the value to a negative one (e.g. -1700). However, the
condition was wrong (double negative), and doing that placed the buttons
in an unreachable area.

PR: 216342
Submitted by: Greg V <greg@unrelenting.technology>
MFC after: 1 week