of_pci: use of_property_read_u32_array()

of_pci_get_devfn() and of_pci_parse_bus_range() somehow didn't use
of_property_read_u32_array() though it was long available, basically
open-coding it.  Using the modern DT API saves several bytes and
also adds some prop sanity checks as a bonus...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <robh@kernel.org>