dev/ofw: Fix ofw_fdt_getprop() return values to match documentation

Fix the behavior of ofw_fdt_getprop() and ofw_fdt_getprop() functions to match
the documentation as the non-fdt code.

Submitted by: Luis Pires <lffpires@ruabrasil.org>
Reviewed by: manu, jhibbits
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D15680