classmate-laptop: Fix sparse warning (0 as NULL)

Fix the following sparse warning:

classmate-laptop.c:523:61: warning: Using plain integer as NULL pointer

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>