HID: betop: add drivers/hid/hid-betopff.c

Commit fc38a8a66e ("HID: add BETOP game controller force feedback support")
is missing the actual addition of drivers/hid/hid-betopff.c due to my
mistake (I forgot to add the file after fixing conflicts).

Fixes: fc38a8a66e1b ("HID: add BETOP game controller force feedback support")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>