intel_th: Use correct method of finding hub

Since commit 8edc514b ("intel_th: Make SOURCE devices children of the
root device") the hub is not the parent of SOURCE devices any more, so the
new helper function should be used for that instead of always using the
parent. The intel_th_set_output() path, however, still uses the old
logic, leading to the hub driver structure being aliased with something
else, like struct pci_driver or struct acpi_driver, and an incorrect call
to an address inferred from that, potentially resulting in a crash.

Fixes: 8edc514b

 ("intel_th: Make SOURCE devices children of the root device")
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>