ums(4): fix incorrect mouse button reporting via evdev

when ums driver receives non-mouse HID report. This results in
unexpected button release event. Reuse existing sysmouse logic to
keep button pressed.

Reviewed by: imp, wulf
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45838

(cherry picked from commit d6888e211b07be79dd8de3145ac41b365f482ae8)