HID: bpf: return non NULL data pointer when CONFIG_HID_BPF is not set

dispatch_hid_bpf_device_event() is supposed to return either an error,
or a valid pointer to memory containing the data.

Returning NULL simply makes a segfault when CONFIG_HID_BPF is not set
for any processed event.

Fixes: 658ee5a6 ("HID: bpf: allocate data memory for device_event BPF program")
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>