HID: hid-lg4ff: use DEVICE_ATTR_RW macro

Use the DEVICE_ATTR_RW macro to reduce boiler plate and move the
attribute declaration to get rid of function signatures.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>