HID: hid-logitech: Compute combined pedals value

Add support for computing a combined accelerator/brake axis for wheels
which don't contain combined data in their HID stream.

This includes DFGT, G25, G27, G29 and Wii-Wheel.

Signed-off-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>