thunderbolt: property: Fix a missing check of kzalloc

No check is enforced for the return value of kzalloc,
which may lead to NULL-pointer dereference.

The patch fixes this issue.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>