wsp: Add sysctl tunable for Z-Axis inversion

This adds a new sysctl to Wellspring Touchpad driver for controlling
Z-Axis (2-finger vertical scroll) direction "hw.usb.wsp.z_invert".

Submitted by: James Wright <james.wright_AT_digital-chaos_DOT_com>
Reviewed by: wulf
PR: 253321
Differential revision: https://reviews.freebsd.org/D28521