test: dm: test-fdt: Use fdtdec_get_int() in dm_check_devices()

The current fdtdec_get_addr() takes into consideration #address-cells
and #size-cells for "ping-expect" property which is clearly neither.
Use fdtdec_get_int() instead and return negative one in case the
property is not in DT or the platform under test is not DT based,
i.e. mimic the current fdtdec_get_addr() behavior.

This fixes ut dm dm_test_bus_children test.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>