HID: hid-lg: Allow for custom device-specific properties to be stored in priv drvdata

This patch adds support for custom device-specific properties which can now be
stored as private driver data and read/saved using hid_get/set_drvdata().

Signed-off-by: Michal Malý <madcatxster@gmail.com>
Tested-by:  <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>