HID: elecom: add support for EX-G M-XT4DRBK trackball

This patch enables the 6th button on the ELECOM EX-G M-XT4DRBK
trackball mouse.

This is a left handed EX-G variant which only comes in a wireless (D)
model. It has a total of 6 buttons but one of these is not available
because of how the HID descriptor is configured.

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