HID: elecom: rewrite report fixup for EX-G and future mice

This patch rewrites the mouse report fixup used for the DEFT and HUGE
elecom trackballs in order to make it generic enough to fix other
elecom mice with similar issues. This patch also uses this new report
fixup function to fix the Elecom EX-G trackball which has 6 physical
buttons and a similar issue to the other two mice.

Elecom's track record has so far shown that they like to re-use the
same report descriptor for multiple different mice regardless of the
number of buttons the mouse has. This means that the missing buttons
on multiple mice can be fixed in one function without introducing
phantom buttons which would in turn cause the number of mouse buttons
to be misreported to userspace.

This patch drops the very verbose report descriptor "diff" comment for
a more abridged yet hopefully just as informative generic version.

Signed-off-by: Tomasz Kramkowski <tk@the-tk.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>