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.

Signed-off-by: YOSHIOKA Takuma <lo48576@hard-wi.red>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>