HID: logitech-hidpp: check WTP report length

Malicious USB devices can send bogus reports smaller than the expected
buffer size. Ensure that the length for WTP reports is valid to avoid
reading out of bounds.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>