fdt: check for fdt errors in fdt_create_phandle

fdt_create_phandle() was ignoring errors from fdt_set_phandle().  If an
error occurs, print an error message and return 0, which is an invalid
phandle.  We also need to change the return type for fdt_create_phandle()
to indicate that it cannot return an error code.

Signed-off-by: Timur Tabi <timur@freescale.com>