thunderbolt: Fix possible NULL pointer dereference in tb_acpi_add_link()

When we walk up the device hierarchy in tb_acpi_add_link() make sure we
break the loop if the device has no parent. Otherwise we may crash the
kernel by dereferencing a NULL pointer.

Fixes: b2be2b05

 ("thunderbolt: Create device links from ACPI description")
Cc: stable@vger.kernel.org
Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Acked-by: Yehezkel Bernat <YehezkelShB@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>