[RFC] Fix OF_finddevice return code for FDT

[I had posted this to freebsd-ppc_at_ and freebsd-arm_at_, did not see any
comments, posting to freebsd-current_at_ to see if there is any interest
or comments]

While reviewing the previous FDT patch, nwhitehorn_at_ noted that the
return code of OF_finddevice was not correct in case of FDT. According
to the 1275 standard, we should return a phandle value of -1 in case
of error, but the ofw_fdt_finddevice implementation now returns 0.

The attached patches fixes this in the FDT code, and makes changes in
the callers to check the return code correctly. Since most of the
callers are in ARM, any testing on ARM would be really appreciated.

Thanks,
JC.