libata debugging: Warn when unable to find timing descriptor based on xfer_mode

ata_timing_find_mode could return NULL which is not checked by all
low-level ATA drivers using it and cause a NULL ptr deref. Warn at least
so that possible issues can get fixed easily.

Signed-off-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>