HID: hid-logitech: Introduce control for combined pedals feature

Introduce a dev_attr which can be used to combine the accelerator
and brake pedals into a single axis. This is useful for older games
which can not handle seperate accelerator and brake.

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