of: unittest: Fix device reference count leak in of_unittest_pci_node_verify

In of_unittest_pci_node_verify(), when the add parameter is false,
device_find_any_child() obtains a reference to a child device. This
function implicitly calls get_device() to increment the device's
reference count before returning the pointer. However, the caller
fails to properly release this reference by calling put_device(),
leading to a device reference count leak. Add put_device() in the else
branch immediately after child_dev is no longer needed.

As the comment of device_find_any_child states: "NOTE: you will need
to drop the reference with put_device() after use".

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 26409dd0 ("of: unittest: Add pci_dt_testdrv pci driver")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>