HID: elecom: add support for EX-G M-XGL20DLBK wireless mouse
Enables three buttons (Fn1, Fn2, and Fn3) on the ELECOM M-XGL20DLBK
wireless mouse.
While this mouse is EX-G brand, report descriptor is a bit different
from EX-G trackball mouse. To enable extra buttons, report should be
rewritten in a similar way to trackballs, but with different position
parameters.