thunderbolt: Refactor tb_acpi_add_link()

Convert while loop into do-while with only a single call to
acpi_get_first_physical_node(). No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>