usb: wpanusb: Fix crash in the sample

Patch 8af3df35 changed logic and did not take into account 2 extra
bytes.

...
  Precise data bus error
  BFAR Address: 0x766e4900
***** Hardware exception *****
Current thread ID = 0x200004e8
Faulting instruction address = 0xddf4
Fatal fault in thread 0x200004e8! Aborting.
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>