libata: Check log page directory before accessing pages

When reading the NCQ Send/Recv log it might actually not
supported, thereby causing irritating messages
'READ LOG DMA EXT failed'.
Instead we should be reading the log directory first to
figure out if the log is actually supported before trying
to access it.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>