Fix the following sparse warning:

classmate-laptop.c:523:61: warning: Using plain integer as NULL pointer
Signed-off-by: NLad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: NThadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: NDarren Hart <dvhart@linux.intel.com>