sysmouse(4): Fix ums(4)-style T-axis reporting via evdev protocol

• Do not report T-axis wheel events as button presses
• Reverse T-axis to match Linux
• Remove wrong comment. T-axis buttons state should be checked by level not by edge to allow continuous wheel tilt reporting

Reviewed by: gonzo
Approved by: gonzo (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12676