HID: waltop: fix Media Tablet 10.6 inch zooming

Fix zoom controls of older version of Waltop Media Tablet 10.6 inch, aka
Genius G-Pen M609. This should make it supported on par with the newer
version, aka Genius G-Pen M609X.

The fix is extrapolated from the behavior of the newer version (Genius G-Pen
M609X) and both older and newer versions of Media Tablet 14.1 inch (Genius
G-Pen M712/M712X) and should work with high probability.

Signed-off-by: Nikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>