fdt: Correct return value in fdtdec_decode_display_timing()

This should return a non-zero value if there is a missing property. Update
the return value accordingly. The only expected error is -FDT_ERR_NOTFOUND.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>